I guess that why im getting bopped sometimes. Only 30f of armor. I dont get any armor from 21d4u4? Can this string not take advantage of the cancel armor properties?
It can, but it depends on how fast you cancel into it, and how much blockstun your opponent is in.
f121 has a ton of blockstun so by the time he comes out of it, your 30 frames of armor have expired.
It works very well in the situation that fox described. When you d3 someone, their natural reaction is to block it, and do a d3 of their own, to try and counter hit you if you press something.
Normally in this situation, you would do EXdb2u to armor through their d3 on a read. But if they don't counter poke, they will block your db2u and you'll be -12 which is obviously very bad.
With this tech, you can do EXdb2 rc 21d4 and then hit confirm into a db2d for a 30% combo. If you guess wrong and they don't counter poke... well who cares, they are still stuck dealing with your 21 pressure and you are free to mix them up with frame trap, throw, EXdb1 etc...
That's a hell of a lot better than being -12 on block if you guess wrong.
Here's a video of this in action: