Actually your reins should not have to be 'short' or 'long' while jumping. You should not have to shorten or loosen them while going over. You should approach the jump having your reins the way they normally are (light contact but not pulling the horse's mouth, but not loopy either) then while going into jumping position you move your hands forward to give a release. In other words, move your hands forward on the horse's neck, but do not tighten the reins. Doing this ensures that you do not catch the horse in the mouth and still have control when you land the jump.
So basically you should
see in pictures is a slightly loose (but not loopy) rein and the hands forward on the neck. When your instructor tells you to loosen the rein, she probably means give a bigger release (move your hands farther up the neck). But I have to warn you: do not ever, EVER purposely shorten the reins over a jump or you could catch the horse in the mouth and possibly hurt them.
Good luck. I hope I was helpful.