Hit Stun Deterioration
Figuring out a way to fight against Infinites has been a struggle for Capcom since the days of X-Men: Children of the Atom, the first entry in Capcom's Marvel series of Fighting Games. They have never been able to quite figure out a way to prevent them, and they've tried everything from the Flying Screen system in X-Men Vs. Street Fighter to the "dizzies" in Marvel Vs. Capcom 2. None of them have proven to be successful and, to this day, infinites still are found in every one of these games.
However, one system was introduced in Tatsunoko Vs. Capcom: Ultimate All-Stars that borrowed from games from other companies like the Guilty Gear series: Hit Stun Deterioration. The concept behind this system is that, as a combo continues to go along, the Hit Stun caused by attacks slowly but surely becomes shorter and shorter and shorter until two moves, no matter how quickly they follow each other up, are not fast enough to actually combo on the opponent.
This is the system they have implemented in Marvel Vs. Capcom 3. And not only is it used, Hit Stun Deterioration is very strong in this game! In TvC: UAS, it took a while for Hit Stun Deterioration to really kick into effect. In Marvel Vs. Capcom 3, however, combos consistent of single hits and no Hyper Combos will rarely reach 30 hits thanks to Hit Stun Deterioration.
However, Hit Stun Deterioration does not seem to be uniform. In fact, as time goes on I'm sure we'll understand it more and more, but for now exactly how Hit Stun Deterioration works is all theory-based. So understand that none of the following is gospel nor fact, but a gathering of theories and guesses as to how Hit Stun Deterioration works. In fact, if all you care to know about Hit Stun Deterioration is what Hit Stun Deterioration is, just take this away:
Hit Stun Deterioration prevents infinite combos because, as the combo goes on longer and longer, the Hit Stun caused by moves grows shorter and shorter so that, eventually, the opponent will escape your combo.
However, if you want to read about how Hit Stun Deterioration might work, read on.
The Theories of Hit Stun Deterioration
This will be the only part of the guide where the voice will be spoken in first-person. The reason is because most of this is theory and I have no empirical proof just yet as to how it works. It may even be properly described in the excellent Brady Games guide on Marvel Vs. Capcom 3, I don't know yet, but this is my take on how it works for now.
Hit Stun Deterioration (HSD) has to be a hidden number behind the scenes that increases as a Combo goes on. As the number increases, the Hit Stun shortens. This much is probably agreed upon by everyone. But from my experience, there are three main concepts that I believe hold to be true:
1) Hit Stun Deterioration does not actually occur as gradually as we think. In fact, I really think some moves may not increase HSD at all. I believe that only certain moves, which I've temporarily called "Triggers," increase the number by a lot more than others and, if we could somehow figure out a way to avoid these moves, you could actually perform fairly long combos. But I actually believe that most moves are Triggers, so the possibilities are very low.
There are some very obvious Triggers in the game: anything that causes a Ground Bounce, anything that causes a Wall Bounce, regular Jump Cancelable moves, and the triggers that cause the biggest deterioration of them all: Light Attacks. I really believe Light Attacks cause a huge amount of Deterioration in comparison to most moves. If you perform around 5 or so Crouching Light Attacks in a row with She-Hulk, the opponent will simply not get comboed by an ensuing standard Chain Combo.
Also easily classified as Triggers are moves that are just deemed too useful by the programmers, typically determined by whether or not these moves can loop: Morrigan's Standing Hard Attack, which can be chained into and linked out of, Felicia's Medium Attack Cat Spike, which can be Buffered into and Linked out of, Amaterasu's Light Attack Thunder Edge (with the Glaive equipped), which can be Buffered into and Linked out of, and other such moves are great examples. As soon as you connect with just one of these, coupled with a Light Attack somewhere, HSD kicks in extremely fast.
2) There are actually two separate Hit Stun Deterioration values: one for the ground, and one for the air. This is something I think is true because, after performing loops on the ground and then launching the opponent, I almost never have any problems with the Air Combo afterwards. If the loop on the ground causes so much HSD so strongly and so quickly and there was only one value, your Air Combo would fall apart almost just as easily... but that is never the case. For example, there is no combo I can do with Amaterasu in Reflector Mode after simply launching the opponent that I can't do if I connect Amaterasu's Hard Attack into Toward + Hard Attack x 6 into Launcher. Same thing is true if I land a loop using the Thunder Edge loop mentioned above: any thing I try after the launch seems to work the same whether I loop first or just try straight from a launch.
However, if I perform a loop where I'm juggling the opponent, since they are airborne, it does affect the HSD of any ensuing Air Combos. After I launch from a juggle situation, they flip out of my Air Combos very quickly. So I definitely believe there are separate Hit Stun Deterioration values for ground and air.
3) Ground HSD builds up faster than Air HSD. Or ground Hit Stun is more easily affected by HSD. However you want to describe it, HSD causes more things to drop on the ground than in the air. There definitely is a lot more leeway when it comes to juggles and Air Combos then anything you can do on the ground. Performing Felicia's Cat Spike loop, for example, you can almost never actually get two Cat Spikes in one combo! Its really hard! Also, all those jumping infinites you can perform on a standing Sentinel lose their ability to combo only after two reps. You can get a third repetition of it, but it's really hard and, after that, the fourth rep is nigh impossible.
Meanwhile, I've actually gotten this combo to work with Doctor Doom: in the corner, perform a Launcher into Jumping Medium Attack (2 hits) chained into another Jumping Medium Attack (2 hits) into his Air Combo Finisher. Land and do Standing Medium Attack (2 hits) Buffered into Launcher and repeat the Air Combo. And then land and do the loop one more entire time. Something like that, to me, should cause Hit Stun Deterioration quickly, but it doesn't seem to kick in all that badly. In fact, even in third Super Jump after the Launcher, you can still combo the second Jumping Medium Attack into his Air Photon Array Hyper Combo. If you do it too slowly, they'll flip out, but the mere fact it can still connect is surprising to me. After that many reps of an Air Combo, I would imagine HSD would be so strong that the third Air Combo would be impossible to perform. But it's entirely possible.
What actually ends up breaking the combo if you just repeat the Launcher into Air Combo into OTG sequence is that once the third OTG Standing Medium Attack connects, the opponent flips out instantly. In fact, in that whole combo, I believe that the only real Trigger is the OTG move. OTG moves cause a lot of HSD, but I don't think Launchers nor Air Combo Finishers cause a lot of HSD at all. And the combo completely avoids Light Attacks all together.
The reason why most relaunch combos cause so much HSD is usually because many of them involve Ground Bounces or probably contain moves that are marked as Triggers (such as Zero's Light Attack Raikousen, which allows him loops without a Ground or Wall Bounce). But that Doom combo listed above uses nothing out of the ordinary, so you can get three reps of the Air Combo.
Avoiding Hit Stun Deterioration
The interesting thing is that, while HSD kicks in fast and combos end quickly, there are actually a ton of moves that get around HSD. All Hard and Soft Knock-Downs, for example, will always cause Knock-Downs. So if you hit an opponent with a Knock-Down move in the air, they will fall all the way to the ground no matter how much HSD has kicked in. The trick really is that most of the time you need to OTG the opponent, and that's the breaking point where HSD kicks in.
Launchers are less affected by HSD than other moves. Though the height at which opponents get launched does get shorter, it doesn't get shorter by much at all during long combos, which allows you to use Launchers as a good means by which to set up landing more powerful moves. Also, Staggers and Crumples all seem to avoid HSD as well. No matter how many hits you've landed, a Stagger or Crumple (theoretically, since it's almost impossible to crumple someone mid combo) will be of the same length every time.
Closing Remarks
Once again, there is no concrete evidence behind any of these theories. They are simply based on what I've seen so far and, frankly, the game is barely three days old so lots of information may be discovered as time moves on. And maybe a lot of awesome Combo Video makers will come out of the woodwork and start figuring it out. Overcoming Hit Stun Deterioration, I'm sure, will become a very fun goal for them and I wish them the best of luck with it. In the meantime, I will be trying to research this topic more and more until I can figure out something far more solid with data to back it up. Until then, however, you'll just have to be happy with the theories presented here.