[Note: The names in the video below may seem confusing to some readers. In the original Japanese version, the character on the left was named Vega, and the character on the right was named Balrog. The names got shuffled around a bit when the game came to America. To avoid confusion, I will refer to them as Dictator and Claw, respectively.]
What exactly happened in this video? Why did Taira, playing Dictator, keep getting hit by Claw's wall dive attack? Couldn't he just block?
The short answer is: no, he couldn't.
What you're seeing happen here is the very simplest form of mixup that exists in Street Fighter: the left / right mixup. Although simple, it is incredibly deadly; the Dictator player has no way to tell which side (left / right) the Claw player will end up on, and so he has to guess which way to block. If he guesses wrong, he is knocked down, and the situation repeats itself. If he guesses wrong a couple of times in a row, he is dead. And that is exactly what happened here.
There are many different kinds of mixups, and each one presents it's own lethal guessing game (of the RSP 2.0 variety). Most are much more complicated than this one, but the basic principle remains the same. They are extremely powerful, because they create a guessing game where the Risk / Reward ratio heavily favors the attacking player. Consider the Claw mixup above - he risks virtually nothing, while standing to gain a substantial amount of damage and a knockdown. The risk / reward on this exchange is extremely high.
In addition to left / right mixups, there are also high / low mixups, throw / counter-throw mixups, reversal / reversal-bait mixups, fast / slow / fake fireball mixups, and some that make use of multiples of these at once. I'll be discussing all of these in greater detail as the blog goes on.

Keep em coming.
Sven02:43 AM CST