It's a pin you use only when raising or lowering the mast. You remove it once the mast is up. The mast shall be free to rotate on the ball, so it can help form a nice aerodynamical shape to your mainsail. Hence the need for rotation control..

Your mast will not hop off the ball, unless something unusual happen. Just tighten the rigging before you go out, and you'll be OK. (you will be OK even if you forget to tighten your rig).

The compression generated by mainsheet, sails and sidestays are tremendous, and they all end up in the mastfoot, so dont worry about the mast jumping off the foot. If your are in need, a screwdriver does the job while raising the mast..