Crumpling to the Ground
Another concept mentioned in the description of Focus Attacks was the "Crumple Stun," which is what the opponent goes into whenever a Level 2 or 3 Focus Attack (or a Counter Hit Level 1 Focus Attack) strikes a standing or crouching opponent. This is a unique Hit Stun animation with some very peculiar properties associated with it.
First of all, let's talk about the Crumple Stun animation. Whenever a character goes into Crumple Stun, it's always the same sequence: the character gets hits, lets out a wail, and falls to their knees. After pausing for a fraction of a second on their knees, they crumple to the floor onto their stomach.
It's actually very important to know that there are two halves of a Crumple Stun because the opponent changes states between the two phases. So let's start by talking about the first phase.
Crumple Stun: Phase 1
During the first phase, from the impact to the point where the opponent lands on their knees and the slight pause afterwards, the opponent is grounded and any hits that connect on the opponent will keep them grounded. Thus, you can actually perform a standard ground combo against the opponent.
Not only that, but Crumple Stuns have one unique trait that a regular Hit Stun does not: you can actually be thrown during a Crumple Stun! It was mentioned earlier that opponents cannot be thrown out of Hit Stun. This is the one exception. Thus, if Zangief connects a Level 2 Focus against an opponent and cancels it into a Forward Dash, he can actually grab the opponent with a Spinning Piledriver during the first phase of the Crumple Stun. All Throws will work: regular Throws, a Command Throw, any Super Combo that throws, and any Ultra Combo that throws.
Crumple Stun: Phase 2
The second phase, where the opponent goes from on their knees to lying on the floor on their stomachs, suddenly has the opponent registered as airborne! That means any move that hits them is treated as though you hit the opponent out of the air. Normal Moves will pop them up off the ground and cause them to land on their feet after the Air Reel. All ground-based Throws will whiff on the opponent. And some Air Throws which grab low enough will catch them off the ground!
Basically, this means that you have to be quick to combo an opponent in Crumple Stun if you want a full ground combo. However, this ability to pop opponents up with Normal Moves can be useful in one main way: Resets (see Resets section in Advanced Techniques). A great example of this is a standard Reset with Cammy: performing a low to the ground EX Cannon Strike on the opponent during the second half of a Crumple Stun will pop the opponent into the air since any Cannon Strikes that connect against an airborne opponent does not knock them down. Since Cammy lands so quickly from the Cannon Strike, she has time to actually dash under the opponent in the air and end up on the other side. Those unaware of or surprised by this trick will most likely be blocking the incorrect direction since you have switched sides on them so suddenly, giving you a combo from the other side.
Extra Crumple Stuns
While Focus Attacks are where you are going to see 99.9% of the Crumple Stuns in the game, there is one other move that actually causes Crumple Stun: C. Viper's EX Thunder Knuckle. Technically, Abel's Ultra I, Soulless, causes Crumple Stun so that he can Throw the opponent for the final hit, but you'll never be able to take advantage of that Crumple Stun.