In freestyle and ultra dynamic riding, upper/lower body separation is needed but don`t mistake this for leading with the upper body which is not the best way to ride.
I think if anything my back hand is fairly good unless i hit bumpy/chopped up terrain. In regular mountain riding my front arm is pretty quiet but I find tat occasionally I twist from the waist.
In freestyle I def am front arm flapper.
I try and keep my front arm to my side but out just a little to guide the landing, but it ends up doing its own thing.
Cutting arms off might be my best option? ;-p