When Sinclair moves back, Jack moves forwards. Maybe he wants to be intimidating. Seems like being able to kill things is his only asset sometimes; maybe being intimidating is the only way he can actually come out on top for once in his horrible joke of a life.

When Sinclair speaks, Jack's face twists.

"Don't use it," he says, "or I'll kill you."

It doesn't even matter that it doesn't work any more, just the attempt would be -- he doesn't even want to think about Sinclair trying to use it, it's making him feel physically sick.