But I Don't Wanna Wake Up Right Now!
After you get knocked down, if you do not perform a Roll Recovery, holding Down on the controller will cause your character to stay laying on the ground slightly longer than normal. You can press Down while you are still in the air and holding it until you land will cause you to perform a Delayed Wakeup. However, keep in mind that only Down works: Down/Back and Down/Forward will not cause you to perform a Delayed Wakeup.
Once you perform a Delayed Wakeup, your character will remain grounded for a short period of time. After that time has elapsed, your character will automatically get up as normal. This allows you to throw off the timing of your opponent's attacks if they try to pressure you with projectiles or attacks right when you get up. However, during that entire time you are performing the Delayed Wakeup, you are actually vulnerable to all low attacks! So you can be hit while lying on the ground. Even jumping kicks can be performed low enough to hit you will lying on the ground.
However, any fear of getting hit out of your Delayed Wakeup can be mitigated just by holding the Block button. If you do this, you can perform the Delayed Wakeup with no punishment, as any attack that connects on your while you are lying on the ground will automatically be blocked (except for Jump Attacks, since you need to stand Block those).
You can interrupt the Delayed Wakeup at any time simply by letting go of Down or by pressing any of the four attack buttons. This will cause your character to end his or her Delayed Wakeup and start rising like normal.