As I watch these videos I notice that virtually all of them put a hand and/or forearm down on every turn. I have long taught myself to not put an hand down because 1) I don't want a dislocated finger, wrist or shoulder and 2) I don't want to rip up my clothing.
However, to get anywhere near as low as these riders is it a forgone conclusion that you must put a hand down on toeside and heelside?
When I watch the hardbooters I admire most, the best ones just barely put a hand down occasionally. One HB in particular appears to have a magnetic repulsion between his hand & the slope. The closer his hand gets, the repulsive force seems to increase as the inverse square law.