The hinge bolt that you are working with is the pivot for the arm. By tightening the bolt you increase the friction holding the arm in place. Several blog entries of other sites have mentioned this in the past. Since the only real time that the arm is moved in this way is when you are replacing the blade this is not a serious problem and in fact has been used in the past as a way to hold the arm up while threading the blade through the project. Personally I have a short piece of wood attached to a large circular magnet gotten from a magnetron from a busted microwave
I put a piece of felt on the bottom to help remove the possiblity of scratching the arm.
DW