Long Lining is working your horse on the ground with two reins. Most long lining work is done with a bridle, two long line reins and a surcingle but you can use a driving harness or saddle and bridle with two lunge lines.
Why long line your horse:
a training step in between ground work and in saddle work
training a horse to steer, listen to the bit, make contact with the bit and carry themselves in a more collective manner
rehabbing a horse from injury
teach the horse to have better self-carriage
be able to watch the biomechanics of the horse that can only be felt under saddle
improve rider communication (how to get a feel of the horse through using your hands and not having rider body interference)
work out training issues on the ground in a safe manner rather then being in the saddle
building and conditioning muscle groups especially the top line of the horse
use as an effective warm up tool before riding (much more control then just mindlessly lunging)
builds a great foundation for teaching horses to drive
Ground Work
Before teaching your horse to long line it is suggested that your horse has good ground work skills. Your horse should be able to stand still on command and you should be able to walk all the way around your horse with your horse not moving. They should be desensitized to ropes around their legs, neck and especially over their backs and bum area as the long lines could get caught around them or on their legs. The long lines will also need to be switched over their backs to change directions so your horse should feel ok to the feeling of the long lines and to be able to stand still while the lines are moved from side to side.
Your horse should be ok with you working from both of their sides and should lunge confidently in both directions and at all gaits.
Your horse should also give to pressure and be able to back up when asked. I do like to teach my horses verbal commands to help make cues easier to understand while long lining or driving because you do not have leg or seat aids to help you and horses cannot read body language if they are wearing blinkers.
Teaching the leading from behind ground work exercise will set your horse up for success in the long lines because they will feel comfortable with you walking behind them.