Pay attention to yourself next time you ride and you might be surprised.
The momentum from the flick your body creates begins to turn the wheel a small amount, and the forward momentum of the bike in combination with the lean keeps the wheel turned right. By contrast, if you flicked the bike/wheel but did not lean, the wheel would simply straighten itself again.
Even for automobiles this stuff is very simple suspension + steering mechanics. Due to how their suspension & steering components are designed, and assuming they are aligned properly, a car will always follow the road. If the road leans + curves, so does their steering.
Fun fact: when I read about it I had to test and I had to make sure I wasn't deceiving myself so I put one palm in front of the handle on one side and the other one behind the handle on the other side. That way I could ensure I pushed it the correct way.
It worked exactly as advertised (50cc bike so somewhere below 60km/h).
Much simpler than using slow motion camera etc.
I can steer pretty well without hands. It seems as if I must be able to initiate the left lean without pushing the handlebars right.
So why couldn't I also do this when not riding without hands?