I've stalled pulling out of a light going down hill. Please don't tell me you've never managed to stall anything, it's as simple as letting go of a clutch without having enough throttle.
Looks like he's applying more control to his riding than attempting speed records in stupidity. 40mph? On that? In the dark?!? Maybe if it was all level, and from the video you have absolutely no perspective on what's level and what's a steep hill, a larger motor could do it easily I guess, still a lot of control is needed to do that with thin standard tires, I'd be playing it easy too, plenty of rocks right in the middle of the trail to break a tire real quick.
He sounds like he's pushing his rpm on the road but not getting much speed. Maybe a gear ratio thing or he's just still working on the pipe and is just limited.
I also really really failed to see where you made the connection between torque and riding at 40mph on a trail. From what I understand you sacrifice speed for torque, and vice versa.
Above all else your commentary was snotty, it didn't even seem to respond to anything, you b**ch about his torque (which I'm sure he has and you have the vaguest clue to how much since you've obviously never ridden his bike) and then when he corrects you by stating he does have torque you go off about the technicalities in what mountain is named what, and proceed to tell us about how often you ride it. Really no connection to anything.
Remember you're talking about a 66cc motor in its experimental stage for tuning, you haven't got the foggiest idea if the engine was having a problem that day, no idea how much the OP weighs (because you understand that 150 pounds is an easier load than a 250 one, especially for a measly 66ccs) and you don't even know why it stalled in the first place.
For the most part he looks like he was riding just freaking fine when he wasn't pedaling, and since he clearly knows how to fabricate sh*t he probably has a pretty damn good idea behind a well running motor vs one that sucks.
Stop being such a snob.