that is the "secondary hand" coming up, during the last of the trigger travel, to lock the cylinder...and why they are more prone to go out of time.
think of how much faster a pistol wears when it's slide is locked to the frame rather then being allowed to recoil when fired...the SOCCOM pistol had this design feature to cut down on noise when fired with a suppressor, i believe the smith m-59, used with suppressors, also had this "added feature"