I have tried to see how Ambrose does this. This is all about maintaining contact, either side by side or behind. When behind Ambrose often has his arms underneath his opponents arms and without wrapping him up, lifts his arms at the time when the ball is about to come in, This is never seen as chopping the arms, but it disturbs his opponents arms enough to spoil the mark.
When side by side Ambrose has enough body strength to effectively disturb most of the players he competes with and prevent the mark. Ambrose can jump well and put in spoils in packs. ( watch out for those quads Ambo)
By contrast, Harts tends to wrap his arms around his opponent, or spoil over the top, and it costs frees for holding or chopping. Other times he just loses contact on really mobile opponents. Harts is good at spoiling in packs and is potentially a better mark and longer kick than Ambo.