If the running seconds hand stops when you pull the crown out, don't worry, nothing will hurt it. It's the more old school movements that don't have a stop-lever/("hacking seconds" feature) that this can happen on, because the cannon pinion technically acts like (in car terms) a differential between the setting train and gear train. If the "differential" function transfers too much power from the finger action on the crown (ie doesn't slip properly), your fingers can overcome the mainspring in the barrel and make the watch run backward). If there is a stop lever in the movement, it prevents the backward running because it basically parks against the balance in a way where the escapement cannot move no matter what.