To be honest, you don't really need to be concentrating on anyting but the target, at that point.
Once you reach your reference you'll want to be expanding your chest & contracting your back muscles to reach full extension. The idea is to get your drawing elbow as far back as possible, so that your drawing elbow, bow hand & your intended target are all aligned. (See Terry's Clock diagrams for an example)
If I remeber, I'll try & get Jules (The Wife) to get some pictures of me drawing tomorrow. It's a lot easier to see what I mean with pictures! I certainly ain't no wordsmith!