Grab mane and hold the reins with one hand and put the other hand behind your back or on your hip
When going into jumping position hold on to some mane because pulling is a sign your using your friend to hold your self up also jump with no reins so you aren't so Dependant on them to keep you up
When jumping, do a slightly larger release than normal an de grab the mane. Don't make it too big though, or else you'll lose contact with your horse and become unbalanced.
Practice reaching a bit forward and holding the mane a few strides before the take off to the jump, it will teach you where to place your hands. Also, jumping lines with no hands helps to lessen your reliance on your hands and will help position too!
You can reach forward a few inches and grab mane going over a jump to stabilize your hands. This should help you make sure you're not catching your horse in the mouth. Grabbing mane is helpful until you master a more balanced half seat and steadier hands. Hope that helps!
My trainer had us do a crest release. So basically you take the reins and pull them to the opposite side so the double up and kinda make a "bridge" over the horse's crest. This will help you hold the contact, but not bump the horse in the mouth.
When going over the jump put your hands really high up on your horses neck so the reins are loose and then grab a section of the mane to help you feel secure without having to pull on the reins.