Toby,
Think of it this way. When you are standing up, you are supporting your body weight on the leg bones, and not on muscle. The muscles are only used to keep the bones in the leg lined up.
When shooting, the bow grip presses into the base of the thumb directly in front of the radius bone of the forearm. The forearm in turn is lined up with the humerus (upper arm) if the elbow is pronated (rolled in). The humerus then presses into the bow side scapula (shoulder socket). IF you maintain a 90 degree angle between the bow arm and body, it is like standing. The muscles are only used to keep the bones aligned and the arm is like a beam that holds the bow "out there."
Then, when you reach full draw, you get the string side lined up with the bow side -- lining up the bones on the string side of the body with the bones on the bow side -- and it feels like the bow gets lighter in draw weight. The bones of the arms and shoulders are supporting bow weight, NOT muscle.
There are a lot more bones to line up for this compared to the standing up example but the idea is the same.
This is really pretty easy to demonstrate in person but more difficult to describe in words -- at least for me.
Arne