I'll chime in, even though I'm rather low on the learning totem-pole on this topic.
The analogy that has worked the best for me is to think of contact as I would talking on the telephone:
- Slack = static-y connection, so keeping the slack out of the rein and that direct line from elbow to bit makes things clear and reliable.
- "Pulling" contact = screaming into the phone unnecessarily, or even like carrying on two conversations at once. The horse has a hard time discerning an actual rein aid from the heaviness
- Lots of weight = chattering away without engaging the other person in the conversation. The person on the other end will tune out and "lean" on your talking. The horse will start to lean on your hands. Worst case scenario, you get evasion - think about trying to politely end a conversation that you'd rather not have: that's effectively what the horse is doing by ducking behind the bit.
The aim is a static-free connection without pulling, and only enough weight to maintain that as a constant feeling. I find that when I ride to this ideal (of course, being careful not to neglect the importance of riding from behind), Scout really stretches forward and rounds over his topline, develops a nice amount of foam around his mouth and starts the beginnings of breaking at the poll.
I hope that makes some kind of sense... More of the ramblings of one of the consciously incompetent at truly correct riding.