classic helicopter haha

Is flogging like an A-10 Warthog burst good for the sail though? I'm curious as to which would be worse (or likely to damage the sail quicker): flogging or stretching?

The leech is "twisted" when the battens are not parallel with each other, or when the roach (area around the leech) has a different shape from head to foot. The easy way to see this is when the sail is luffing/flogging: without leech tension, the battens move of their own accord; with tension, the battens move like a synchronized swim team. In practice, when you sheet in block-to-block, you would tighten the vang appropriately, such that the same leech tension is maintained when you inevitably ease and re-trim the mainsheet (the boom will move out-and-in (boosting) rather than up-and-down (bouncing)).

You're not missing anything. His vang had no bearing in his first clip. It was all about the mainsheet tension. His 2nd clip where he says he had no vang on....simply bad advice. In that wind condition, the only thing the vang is doing upwind is limiting how high the boom will go in the tack. There are way better clips out there on how to setup upwind. He was barely even hiking and his roll tacks were a bad example, really sloppy footwork, barely any roll. I would expect more from a "top tips" from the RYA.