Short answer it is simply a pin underneath the grip panel on a 1911 that in essence replicates the grip safety being engaged. A lot of folks with small hands like the feature as their grip or hand size if you will makes engaging the grip safety on a 1911 difficult. Back in the day many competitive shooters would also utilize the pin just in case they got a poor grip on a speed draw or recoil type incident. The pin is hidden under the grip panel so there is no witness mark that it has been pinned.