I have very strong hands and I used to sneak up behind people and grab the side of the ribs and press, the person invariably ended up on thier knees.
One day I did this and the guy turned fast to push me away and his elbow caught me in the face, I heard the crack as my cheek bone broke ,funnily enough it didn't hurt . I ended up having to spend 3 days in hospital for an op to fix the bloody thing.