It’s a common design for conceal carry. The hammer is still there just doesn’t have the thumb lever. It’s a double action. Pulling the trigger pulls the hammer back until the release point and the hammer then falls sticking the primer. The trigger will be pulled all the way for each shot with this shaved style hammer. If the hammer still had the thumb lever, the hammer could be pulled back into a “cocked” position at which time pulling the trigger would release the hammer to strike the primer.

A traditional hammer can become caught on clothing or other loose fabric when being concealed and moving around or drawn from concealment.