the dog will instinctively go for your neck...
I once had a "friend " who had a "trained" St. Bernard, biggest I've ever seen to date.
It stayed in an open closet just adjacent to the house entrance where when the owner opened the door, he was between the dog and the entrance.
I was sitting in the room when there was a knock upon the door and the owner went to open it.
It was someone he knew that was very exited and when the door opened, he rushed in past the owner.
The dog came out of the closet, stood up and literally grabbed the guy with his massive jaws encompassing his head and began to drag him back into the closet.

The owner grabbed a short spade shovel (apparently his dog control device) and began beating the dog over the head.
I mean full on whacks.
No effect.
He then used the shovel to pry open dogs mouth to free the guy.
I left.