Half cock notch on a 1911 is to catch the hammer in the case of the main sear failing. It is not a "safety" in the same sense some half cock notches are (Winchester model 94 for example). The firing pin is inertial (does not contact the primer when the hammer is fully lowered), so a blow to the hammer won't cause a discharge.
BTW, on Colt Series 80 pistols, the half cock notch is not captive. You can pull the trigger from half cock, and the hammer will fall the rest of the way, but not forcefully enough to set off a round.
