You need to keep your
bottom arm HIGH as you plant, or else you will continue to
slow down at the end of your approach, have a
late plant, be
under,
not finish your take-off, etc...
Just so that we stay in the right direction, the C position is NOT the goal, it's to finish the take-off and IMMEDIATELY try to swing...the C position happens quite naturally, and looking at it on film can be an indicator of the ROOT problem, which is naturally not finishing the take-off... there are a few possible CAUSES of the root problem, and we must try to diagnose WHY what happens happens. I do not see the head thrown back, and the hands look ok (at take-off), so he is either under or has not been taught/learned to take-off with a complete extension of the toes and ankle. In this instance, it could be both, but I assume that given an appropriate take-off point, he would finish his take-off correctly. (***Let's not look at things on such a shallow level. Look deep into the vault and see what is there, what is not, and then make a decision on how things should change...***)
And now for how to change it: you're lucky enough to have a Summer season, and you should fix whatever you can before the real seasons start, regardless of how it affects your vault immediately. The next practice you have, start with 1 step Jagodins (jump, stay behind the pole, ride it into the pit). Take-off OUT about 1 inch. Now do that maybe 20 more times. Then back to 2 steps. Take-off maybe 1.5-2 inches out now (let's not argue about these numbers in this thread, look at the 'take-off point' thread if you want to discuss them). Do that a few dozen times. Now back to 3 steps, WITH A REAL PLANT, out just an extra half inch or so (don't worry if the mark is perfect at the moment). Repeat 20-30 times. The next practice you have, start with 2 steps, work back to 4 steps until you're completely comfortable at 4 steps. Keep working back the run in this drill until you are at your full run. It could potentially take a few months if done correctly, but you will not take-off under habitually any more (unless you never really did, and the vid you posted was a one-off, and that would mean I'm just sort of wasting my time posting this where nobody will ever read it...
). You would still benefit from this drill if you're take-off was naturally 'out'. DON'T FORGET TO PRACTICE THE PRE-JUMP WITH THIS DRILL ON EVERY JUMP!