One thing to keep in mind is that backing is not a natural movement for a horse, it's not a natural defense or play movement. Also, when a horse backs it does so by moving it's legs in diagonal pairs, the same as when it trots forward, at the same time the head is moving from side to side. So it makes sense then to ask for the back by instead of applying direct downward pressure with the lead rope you build on the side to side movement getting them to shift their weight on the front legs first.
Here's some info that explains to in more detail: How to Teach a Horse to Back Up | eHow.com
When your under saddle, the back comes from stopping the forward motion. In other words, you have you leg on but are blocking the forward movement by setting your reins. There's no pulling or lifting of the reins, they get the release by backing instead of trying to go forward into the bit.