ORIGINAL: Pavomesa
Thanks to this, you can chamber a round in a .45, let the hammer fully down on the round resting in the chamber and them smack the back of the hammer with a rock and the gun will still not fire. (No inertia as the firing pin is pushed out of the way by the hammer.)
The catch is if something accidentally pulls the hammer partially back and then slips loose. It could theoretically cause the gun to fire. By carrying the gun "cocked and locked," the hammer is LOCKED in position and the gun is protected by two safeties.
Actually, what prevents this more than anything is the firing pin block present on Series 80 and subsequent Colts and most any 1911 made in the last couple of decades or so. Hence the third safety, which is deactivated by trigger pull.