I think it could be a shadow of reins or a breastplate. Not sure. AM absolutely uses reins and bits as tools for punishment in this video though.
If you really scroll through and watch the reins instead of the punching, you’ll see that there are points where he is standing in the stirrups and leaning back, using his entire bodyweight to saw back and forth on the horse’s face.
There are few things as pathetic to watch as a man who is pushing 40 assaulting an innocent victim while pitching a overgrown toddler tantrum during sport practice.
No offense to toddlers.