Never heard of it?  You are not alone.  Bound is probably the least known transition by name, although it is utilized often.  Bounds are a catch-all for unified transitions.  Simply put, they are any unified transition with an adjustment step in the middle.  This means that there are 3 main types of bound, one emulating a punch, a pop, and a reverse pop, but with an extra step on the ground.  It could be said that bounds are the redirects of 2 footed transitions, and mastery of them comes through mastering the unified transitions and the redirect.