Backing up is not a natural gait for horses. It is very rare to see a horse back more than 2 or 3 steps on their own. Most of the time, the only reason a horse will back up on their own is in order to get out a situation they don't like and only back up enough to have space to spin and move forward out of the situation, such as being backed into a corner by another horse.
This is in part due to the blind spot that a horse has directly behind them. They cannot see behind them and do not want to back in to a bad place.
It takes a great deal of trust for a horse to back up to where a handler is asking them to. This comes into play when asking a horse to back out of a trailer, back towards a wall or back in any obstacle situation. The handler/rider is basically asking the horse to back off into what might be to them a black hole because they cannot see directly behind them.
Backing up, while not natural, is a great way to get a horse to work their back and engage their hind end, if done correctly.
For a horse to back correctly, their head should be at a natural headset, not braced, thrown in the air, or down between their knees, and they should pick up their hooves, not drag them though the dirt.
When asking a horse to back up, start small with little shifts of weight backwards. Build on the shifts, making sure that once you have started asking for a back, there is no release/reward until the horse either shifts backwards or steps back. There is a huge difference between asking your horse to back and pulling them backwards, either with the reins or with the lead rope.
There are lots of different ways to ask your horse back up on the ground. You can wiggle with the lead rope, put your hands in the air and "pump" the air towards your horse, or using the lead rope by the halter directing back towards the chest. Find which way works best for you and your horse!
Check out this YouTube Video to see some of the different ways to ask a horse to back up!
Check out this YouTube Video to watch a horse backing up in real time and in slow motion!