Get a yoga ball and a set of dumbells. Parallettes or a floor bar can work.
Lay on the ball with yours hips on it. Set your shoulders over your hands as if a pushup position.
Rock forward to go into a supported planche position. Extend backwards to go into an extended open shoulder position.
The whole point is to go from an extended open shoulder position to a deep planche position. You pull and push your body while keeping it horizontally.
You can use either things to slide in the movement. We have these little baby scotters on wheels. We have the gymnasts flex the ball of the foot on top of these. If the bar or dumbells can stay stationary you could use one of those frisbees to move the feet.
Remember those ab wheels sometimes the Wheel of Death? Same thing. Just a little wheel with handles on each side.
Works pull and push strength besides keeping the midline tight if they don't sag in their back.
Really works the deltoid and shoulder muscle necessary to close the shoulder angle pushing into the support.
Got a Total Gym. This can be done with the cables like an uprise. Focus on straight arms. Since it's only partial BW, you can focus on going fast.
You could have a spotter do uprise from a horizontal glide swing like hang. Spotters swings legs backs and gymnasts pushes to support. This can be done instead pushing their legs off a block.
Remember the old stem rises on Uneven Parallel Bars. Set one high, the other low. Feet jump and push off lower bar while holding higher bar, pushing to support.
As other have mentioned, it really helps to work V hold to the bar so you can stay compressed once you lift your toes back to the bar after the glide. It's just not a matter of bringing your toes back to the bar, but keeping them there!
V-ups and leg lifts should be your friends if you want them snapping your toes back to the bar. Ab strength is almost more important than the pull/push strength. Of course, if you can do a bar muscle-up you're plenty strong to kip. Nice to have first. Perhaps a bit extreme, but good down the road for a strong kip that arrives in front of the bar ( besides with the hips as piked as possible ).
If you can't do leg lifts toes to the bar, start from just lying on the ground. Bend your knees if you have or start by doing tuck ups. You can also lie on an angle and do a leg lift with your arms extended ( I set up a cheese mat with a floor bar at the top ).
the last drill I can think of starts with the gymnast lying on their back. Have them hold some sort of weighted stick or bar ( hopefully they won't let go of it ).
lift their toes back to the bar and quickly situp, pulling the bar to the hips with straight arms ( somewhere in there jumping straight arm supports or stem rises should have been done ).
if you have access to a pulldown machine or create the theraband pulling station. attach a bar to the theraband to simulate the skill
