Now, I'm not a jumper, but I think what she is saying is that you're balancing on the reins. It's a really common problem. You're letting him lean on the bit and you're leaning on the reins to keep you steady, instead of letting him hold his frame naturally and using your legs and your seat to balance yourself. When you're holding the reins, you feel more balanced, so you don't worry about your legs, really. When you let go of the reins, you are off balance, so your legs swing, trying to find that security you had with the reins.
I think what she's suggesting is that you have your coach put you on the lunge line to do some reinless work - to make your seat and legs more stable and secure and stop relying on your reins for balance.
Correct me if that's not what you meant - that's just what I noticed about the video.
Oscar's super cute, by the way. :] Best of luck with him!
Aw kk thanks for explaining that to me