It's a form of restraint......Not attack
The dog will grip (bite) while pulling the person down. It continues to do this and modify it's grip until released by the handler.
Unfortunately for the bad guys, they don't wear full body padding, or even padded sleeves or face/throat shields as they do in training..
They still get bruised though, whereas the baddie will naturally have broken skin and bruising while in a hold.
Have you seen the Amsterdam police/army method for releasing their Belgians from a hold? ....They place the dog in a choke hold and wait till it releases.