The weapon is likely to discharge as you grab it, but the idea is to grab it and strike the inner wrist (affecting the nerve system which is involved in carpal tunnel syndrome) ... it is vulnerable where he strikes. If the weapon is a large caliber revolver, the hand grabbing is gonna get burnt, but grab you must, or be the victim. With a semi-auto the hand will be injured when the gun discharges, but grab you must, or be a victim.
If you push slightly on the grab, most semi-autos will not fire. It pushes the slide just enough out of battery to prevent firing.
Everything he says has to be done near simultaneously. If you move your head out of the way too early, the shooter will be spooked.