The shaking and noise is genetic, but the development of the dog and how he learns to fight a human is determined by the TRAINING decoy/helper. When the dog believes his bite is not having effect on bad guy, the dog will go into slight defence and shift to his front teeth and regrip and shake. This is an unsure dog. The goal of the training is to ensure crushing full grip bite in balanced fight drive. Too much prey and you get begging for the sleeve in a high pitch bark. Very undesirable! Too much defence and the dog shows stress and eventually avoidance behaviours.
The same can be applied to obedience where most of these so called tough dogs just fall apart. Why? hmmmm. Same theory. You need high prey since conflict from requiring to do a specific exercise creates much conflict. When you have a dog that has handler hardness, then that conflict will translate into POWER. Same as in the blind. If you just have prey, you get little power/fight.
The fact is, not many dogs can deal with the conflict from both sides of the leash, handler and bad guy/required exercise. that is why you only see dogs being posted and given bites on most videos. That is almost all in prey. There is little or no conflict management/training.
PS. good question. Join an IPO club. If you want to understand dog behavior, there is no better venue.
E-Dog - 2012 WPBTCA National Championships
The time when screech owls and Bandogges howl and spirits walk and ghosts break up their graves.
Act I, Scene IV of William Shakespeare's "King Henry VI"