So I've been working a lot of strength building in my practice recently...and it's really damn hard!
But in order to balance out the flexibility of my spine it needs to be done. I'm used to lifting my sternum and my lower ribs away the pelvis, creating lots of length in the lumber spine for drop backs / kapo etc. So the whole feeling of contracting the body in the opposite direction for jump-backs and jump-throughs / navasana to handstand etc feels very alien.
Nevertheless, I'm really working hard to find this strength and to achieve a balance in my practice.
Found a good video about jumping back the other day which said to keep the head lifted, eyes looking up when you go to jump back. It kind of helps engage the back more I think as well as bringing the weight forward more so the feet can scoot back. I'm visualising a tortoise, back rounding like its shell but neck and chest lifting out and up!!
David Swenson said the same thing about jump-throughs at his workshop, keep the gaze lifted and the hips will follow through the arms. So I spent a good 20 minutes extra today just jumping back and through and I'm so close...This tip could just be what enables me to build the strength to pull off the jump-back.
WORK IN PROGRESS :)
Just another thought...there's kind of the same action in bhujapi, I might practice this a bit more and see what happens :)