It operates the bolt. On a Colt the bolt spring is flat and is split in two. One side operates the trigger and one side the bolt. These tend to break. The Colt hammer has a fixed cam that raises the bolt. The bolt leg is separated into two legs so that one leg can flex inward while going back over the cam when the hammer is dropped. Again, this is a weak spot and the legs can break. On the Ruger there is a plunger and spring. in the trigger guard that pushes the bolt up into its locked position. For unlocking when you start cocking the hammer the hammer plunger pushes on the back of the bolt and tips it down. When the hammer falls, the plunger is pushed back into the hammer so it can slide over the bolt and reset for the next shot. A more reliable system.