I’ve seen Kottas get pretty aggressive with a whip on multiple occasions, esp. when trying to teach half-steps/piaffe. In one instance, the poor mare just shut down and endured repeated hard slashes on her hind legs because she couldn’t grasp the concept of putting a hind leg up underneath herself instead of behind her. In another instance, he laid into a friend’s horse when it wasn’t picking up its legs fast enough - horse reared in response and went up so high we were all afraid for an instant that it was going to go over (and my friend was on its back!). In another episode, he went after my horse and learned very quickly you don’t do that to my horse without seriously endangering your own life (my horse was big and powerful and he did not tolerate unfair or rough handling - his explosions could register on the Richter scale). That was the last clinic my horse did with him - it wasn’t worth the risk to either horse, rider, or clinician.
Bear in mind that Kottas removes boots/bandages from hind legs when teaching a horse piaffe, so the whip lands on bare skin.
Also, he got pretty adamant that people were not allowed to video the clinic or take pictures, even of their own horse. We thought at the time it was because he was trying to market his own line of videos but now understand that it was because he didn’t want evidence of his “strong” use of the whip to be caught on film.
I will also add that he starts out very gently and methodically, with light touches/taps to teach the concept, but will and does escalate to harder taps, raps, even slashes if the horse doesn’t respond in the way he wants it to.