Commitment to a leap, vault, or other movement that involves jumping requires a commitment to the jump. If your body is only half into it, failure is almost assured.
That said, I have problems with committing my weight to all-forward momentum for jumping. I've tracked it down to the fear that my legs will hit the bar (or whatever I am practicing on), and stop, but my upper body will keep going, and I will end up doing something awkward (and quite probably painful) as I try to land
through a solid object.
Short of finding someone to watch me as I try to leap over progressively higher obstacles, and let me know how much I made it by each time, I've been stuck on how to work my way up to "maximum height" without feeling uncomfortable/afraid. Until tonight.
I realized that I could take tissue paper, maybe even
wet tissue paper, and (because of how its stored on rolls, going out to extreme lengths) drape it between two poles, at some height; then, try to jump over it. If the (wet) tissue paper doesn't tear, I know that I didn't hit it (and I can leap several times to know that it's a measure of my general ability, not just how I did that one time), and I can raise it a bit before continuing to experiment. If, as will happen eventually, my feet or any other part of my body intersect the (wet) tissue paper, my motion will not be impeded.
I also thought of using normal paper, since the tissue paper could be a bit too expensive (even after cutting each roll to make several strips), but normal paper doesn't stretch quite as far. That's when I thought of using
ribbon 
Find two poles. Take one end of the ribbon; if you tie it tightly, it might resist your fall, but if you tie it loosely, it might slip down along the pole as time passes. You don't need to worry about either of those, since you won't even be tying it. Don't even loop it around the pole. Just tape it to the side, and make sure the tape isn't so good that it'll stick like glue.
The material itself doesn't need to give way, so long as it doesn't hold on to the anchors. Same effect: the ribbon comes down with you, you don't get hurt, and you know that you can't (always) jump that high. It's the best of all possible worlds.
I suppose you could also use string, which has a better chance than ribbon of breaking, but I wouldn't rely on it (for fear that it
wouldn't break), and if you're already taping each end to a pole you may as well go with ribbon instead (more durability).
I only had this idea tonight, and I haven't had a chance to try it out yet, so: apply common sense and the usual caution. If you've tried something like this, or it seems like such a neat idea that you couldn't wait to see how it worked for anyone else, please reply to share your experiences.