You can find most of your answers over at
HalfCocker.com, but a quick sum up to the question I definitely know the answer to: The pin goes through the bolt down thru the slot in the upper surface of the lower tube, and when pushed backwards by the ram on the first half of the trigger stroke, uses the slot in the hammer to push it back to cock it. Basically the whole point of halfblocking (besides getting rid of a large chunk of metal out of the upper tube) is to get rid of the backblock and cocking rod. The slot in the hammer and the pin through the bolt replace the connection that the backblock used to provide.
Clear as mud?