[ Maybe some day Jack will find out about Xander's basement and Hawaii shirts and really pathetic suicide attempts, but today is not that day. Partially because Spike doesn't really have all those details either. Thigh is about as good as anything and he makes an effort to drive the stake a little further into the muscle, putting his weight behind it as he lets himself fall with Jack. Being on top is infinitely preferable so he'll scramble for dominance, attempting to straddle the other so he can pull the stake out and penetrate some other non-lethal part of him. ]