Dave,
Moebow's explanation was quite good, actually. I'm going to add a bit to that. To feel what muscles should be activated during the draw, try this.
Hold your hand and draw arm as if at full draw. Have someone stand behind you and push on the elbow of your draw arm. Your job is to push back against their hand. If done correctly, your lower trapezius (at the end of your scapula and near your spine) should be activated. You'll also notice that your scapula feels like it's going to pop out.
For me, it helps to think "move the elbow back" instead of "move my draw hand back".