Question regarding the new phys bone system. I want to have a phys bone on my avatar that only reacts to player's grabbing/moving it around...but does NOT move on it's own.

In other words when I move around it has 0 actual moving parts on it and does not move around like dynamic bones did. However if a player grabs it and moves it it will react to that grab and pull accordingly. How exactly do I d this in the phys bone settings?