Long and low in its true sense requires a great deal of balance and strength to be established in order to maintain that working 'frame'. You are asking the horse to keep its hind legs active and engaged, lift and swing over the back, remain light through the forehand while the head and neck is being carried low. As equitate said, it is a great way to test the quality of the work.
Unfortunately, most people will just drop the reins and when the horse lowers its head they cheer and call it long and low. Sure it is good for a horse to learn to stretch but this doesn't build topline and if anything, causes tension in the back and haunches.
A young horse can hold a few steps in this position with a good rider but will quickly fall on the forehand when the hind legs lose the strength to carry.
It is a very difficult position for a horse and until it is working comfortably into contact with a neutral head/neck position, long and low is about as useless as a chocolate tea pot.
If you're just slopping about on a loose rein for a relaxing ride then yes, great for the horse to stretch forward and down but that is not to be confused with long and low.
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