That is the disconnect. When the pistol is ready to fire the hump fits inside of that half round cut in the slide. after firing the slide moves back and the hump gets pushed down becuase it is no longer in the hump. When the hump gets pushed down the whole trigger bar gets pushed down and disconnects fron the striker safety lever, mean while the sear spring realigned the sear and safety lever back to resting position (the trigger bar is still disconnected). Then when you let the trigger go the trigger spring pushes the trigger forward which realigns the two nubs on the safety lever and trigger bar. The slide is back forward and the bump is in the hump.