When you do groundwork you can easily see the lateral bend in the body when you film your horse from above.
When Toronto came to me a few years ago he was a very left bended horse. That meant his left side was the concave, hollow side that contains short, stiff, strong muscles. His right side of the body was the convex, stretched side contains longer, more supple, weaker and less developed muscles. Therefore he would easily bend to the left:
The first step in straightness training is always to make the horse bend equally to the left AND to the right with an equal and mainly sideways bending of the spine from the 1st neck vertebra to the last tail vertebra.
By stretching the short inside muscles and contracting the longer outside muscles in groundwork, a horse can create proper lateral bending to the difficult side. But just like it is difficult to learn to do splits in one day for us humans, the horse will need time to lengthen its short muscles and to contract the long muscles!
A crooked horse bends easily to one side and with more difficulty to the other side. A straight horse can bend easily to both sides and has the correct amount of bending on both sides.