Boingy boing seems to be one of those tricks that takes a short time to learn, but a lifetime to master…so to speak, anyway.
I’ve thought I “had it” numerous times only to figure out that I didn’t have it down pat.
But now I think I have it. The trick for me is to leave your throw hand wrist completely limp, and do the boing-ing with your forearm, elbow, and bicep. When I used my wrist instead, which is the first thing you try, I was always getting ahead of the timing and there wasn’t enough slack in the motion for the yoyo to really go as far as possible in both directions. With the wrist limp though, makes all the difference for me.