Willow, I would not consider that pushing a horse too hard. That was an EXCELLENT run, I've watched that video so many times. By "Pushing your horse too hard", I would think that would be spurring them until their sides bleed, running the without proper flexing and warmup (Running cold), yanking and pulling on your bit so you tear up and harden your horses mouth, and a horse with a lot of heart is capable of running themselves to death (This doesn't happen much, but it IS possible)
A properly trained pole horse needs little help from you. It will do everything willing from light signals.