The manual says that its not a mechanical safety and should not be used for carry purposes.
So what's it there for? What would be the harm for carrying that way?
A half-cock notch should definitely NEVER EVER be used as a mode of carry. EVER. REPEAT - NEVER. The purpose of the notch is to arrest the fall of the hammer if it slips from your thumb while cocking, or in the event of primary sear failure.
Note that the half-cock notch is strangely located so that the hammer is back far enough that if the half-cock notch fails, the hammer fall still generates plenty of energy to set off the primer and fire the weapon. This is exceedingly dangerous.
Also, when in half-cock, the hammer is exposed and can be sheared rather easily. Half-cock leaves the hammer in a position such that nothing protects the hammer from a blow to the rear which can drive it forward, whereas full-cock is in a position that the hammer is mostly protected by the beavertail in the event of a blow to the back of the hammer. A blow to the face of the hammer while it is in full-cock will simply drive it back and it will re-engage the full-cock notch on the way down.
Jeff Cooper used to tell of a guy who sheared half-cock on his pistol with a blow from his elbow.
If you want to shoot yourself, go ahead, but keep in mind, when you carry in a known unsafe condition, you also endanger everyone around you.
S&W later discontinued the half-cock notch on its pistol hammers after they introduced their firing pin block.
Colt changed from a half-cock notch to a safety shelf which puts the hammer barely above the back of the firing pin so that if it falls from the safety shelf the hammer does not transfer enough energy to fire the weapon.
Colt's manuals used to say "never intentionally put the hammer in the half-cock notch" or words to that effect.
BE CAREFUL OUT THERE!!
