When you try to block a fireball, have you noticed how your character starts blocking before it makes contact? This phenomenon is known as proximity blocking.
Dating all the way back to Street Fighter II, every attack has its own effective range which triggers proximity blocking. If the opponent holds Back while within that invisible range, they’ll stand still in defensive stance instead of walking backward.
Using this knowledge to your advantage is a key element of advanced footsies. The easiest offensive application is whiffing a jab while your opponent walks back and forth at mid-range. This will momentarily cause them to get stuck in place.
Now that might not seem like much of a difference-maker, but when they’re focused on the tiniest details, it’s enough to completely throw off their bearings – however briefly. Especially when their eyes are fixed on your character, they might even lose track of where exactly their own character is located. If they try to walk backward and their character remains in place, they may not realize how close they’re standing.