Not sure really, cause I think there is/was a height threshold for a bike jump to be fatal, so it could be just not hitting that threshold. Another thing is that some players say that landing on the rear wheel could help with not falling off.
Timer should start when you can move your character, but connection between players, and clever coding by R* can mess that up sometimes. So to be safe start your timer right when you see the Green banner comes down at the start of the heist.
Very useful videos for beginners, back in the day when I first started getting into OG HEIST, I had a hard time finding useful videos. But your video will help a person to understand how to act on each role, well done.