While traditionally these type of jaws are used in the compression mode - depending on the bowl shape, the jaws can be used to grip in either the contracting or expanding mode.

To ensure best grip, the bowl should have a positive or negative flair towards the edge of the bowl.

Try and avoid straight sided bowls where the grip will be less efficient. Best cutting technique is to have a cutting action with the chisel such that much of the force is directed towards the headstock, not across the face.