To my knowledge, b.21 needs a dash buffered d.b.f.3 to work but really theres quite a few set-ups for it. Doing a juggle sweep will cause the same situation with a different timing (b.23 d.2 d.3 for example), ending it with f.1 at max height will let it happen, EX venom upper in the corner is the most famous example... Hell, if you read my normal throw vs command throw I list a few examples there to make use out of.
You can command grab them perfectly from the distance after a throw and they have next to no buffer time against you, can't roll away, and the like. So you can technically chain trap throws into command grabs for positional resets and whatnot, like doing j.d.3 b.1 f.grab on block into command grab and going from there. His normal toss is only 10 frames of start up, so imagine a command throw type deal that's twice as fast but not comb0-able. Not bad imo.