good morning,
planting under is a product of the run... the correct distance from 6 steps to the takeoff is the key to eliminating "UNDER"..
www.oneapproachrun.com print the chart and check where you are at the six step from TO.. compare that to the chart "MID" for your grip and jump height.
if you are "OUT" by those numbers there is a very good probability you "stretching" and can't get the foot "under" your body mass over the last 2/3 steps.. creating "under", late, big pole bend before the takeoff (none free) etc...
what is "counter intuitive" but true 99% of the time is: one .. If you are "under" on a vault you should move your steps FORWARD to hit on the correct six step "MID" so you can "get the feet down" quickly and out at takeoff.
Two: The other "solution" is to "push" (slight lean forward) harder out of the back/start of the approach which allows you to have the same speed but a slightly longer stride getting you to the correct "MID" with go speed and posture, allowing for 6 increased frequency steps and a "OUT", "impulse", jumped takeoff.
dj