In which action check does the hammer move down upon squeezing the trigger?

The correct response is single-action check. In a single-action check, the hammer is in a cocked position prior to the trigger being squeezed. When the trigger is pulled in this configuration, the only action that occurs is the release of the hammer to strike the firing pin, thereby firing the round. This mechanism is straightforward, as it involves the trigger solely performing the action of releasing the hammer, without the necessity of cocking it again.

In contrast, during a double-action check, pulling the trigger not only releases the hammer but also cocks it back into position before it drops forward. This leads to a longer and heavier trigger pull since the process involves both cocking and releasing the hammer. The decocking check involves safely lowering the hammer down without firing, which is different from actually squeezing the trigger to fire, while a slide stop check pertains to maintaining or releasing the slide on the pistol, not directly involving the hammer's movement. Hence, the mechanics of the single-action check are distinctly characterized by its simplicity in trigger function, specifically involving the hammer's movement upon trigger pull.