Our sails can have different trim at different heights. This is what twist is about. Twist is not a bad thing, infact sails have twist built into them by sail makers to reflect average conditions.
A sail with a lot of twist in it would be one which has an open upper section but a closed lower section. This is in contrast to a sail with only a little twist which would have a much tighter leech.
So what does twist achieve? We use it to set up our sail to the prevailing wind gradient so that we can take advantage of the stronger winds higher up and the more open apparent wind angle that this produces.
We can add twist to our sails to give us more speed, whereas less twist will allow us to point higher. When adding twist we should ensure that the mainsail and the jib both have the same amount of twist.
A common scenario is that twist is induced as we come out of a tack so that we can get up to speed quickly and then flattened out once we are up to speed.
Once the wind has built we can depower the sails by increasing the twist. This works because wind spills from the top of the sail.
How to Control Twist
In moderate or heavy waters the primary twist control for the mainsail is the mainsheet which will increase twist when eased. In high winds and smoother water it is preferable that power is spilled by flattening the sails.