And who mentioned any thing about normal conventional methods being cruel?
Karl giving some piaffe/passage training
And another showing how a person on the ground can encourage forward movement by just holding a whip - no leg tapping - and also shows how too much forward encouragement will make a horse tense and agitated so it runs away from the hand
Both of these horses have reached the level of understanding of collection and have the muscle for it to be able to perform the movement which should follow naturally with the right cues
A horse that lifts its legs up to the cue of having its legs tapped is akin to trick training