Invulnerable frames on backdash. - Super Street Fighter 4 Note - sf4answers

Golf_Rat on Twitter posted the following video where he plays Guile and throws an EX Sonic Boom at Ryu, who backdashes and does not get hit or take damage from the Sonic Boom (@ 0:49):

Which led to the subsequent tweet:

@sf4answers Check out this video. Sonic Boom goes right through Ryu. No damage??? WTH.

The reason for this is that the first frames for most of the characters backdashes have about eight frames of invulnerability on startup.  El Fuerte one of the exceptions, with 6 frames of invulnerabilty, as well as Dan, who doesn't really have a dash as much as a run and is not invulnerable during that run.

Here is a video showing the concept with Zangief and Cammy, where Cammy tries to sweep Zangief repeatedly but whiffs, due to the invulnerability frames on backdash:

It's also why you will see players backdash on wakeup after being knocked down, in order to evade opponents that are too close (jumping in or otherwise), as seen here in the match between Justin Wong and Vangief in the top 32 winners bracket of Evo 2010 (Justin's Rufus hits Vangief's Zangief in the air and Vangief immediately dashes back on wakeup):

However, there is a risk associated with this, even though you might evade the first move, there is a strong possibility that the character will use another move which will hit you.  This results in you being knocked down again; in the second part of the backdash animation the character is airborne.