Pinned armies

Can someone please explain how this works? I cannot figure it out.

I think/thought it to be the following.

You have an army/land force in a province. You want to send other forces to that location THAT ARE NOT BEING BUILT (this to distinguish from when building sticking a pin where you want the army to end up).

I thought the process was something like this. You locate the army force you want others to coalesce around. You pin that army on the top of the screen bar with the pin symbol. You will then see a pinned armies box in which is shown the army with a number. You go to the force you want to move to the pinned army's position and click on the pinned army. But no the force you want to move does nothing so how do you move the thing then????