From The Handbook of Sailing
Sometimes, when winching, the coils of sheet on the barrel become crossed (known as a riding turn). This is usually a result of taking too many turns on the winch. It cannot be undone unless the tension is removed. Another line is first tied to the sheet, between the winch and the sheet lead, using a rolling hitch. The new line is then taken around a spare winch and wound in until it is taking the strain from the first winch and the riding turn can then be released. If you find that riding turns are occurring frequently, you should check the angle of then sheets to your winches.
For more hints and tips on sailing techniques for both the beginner and experienced sailor, purchase Bob Bonds The Handbook of Sailing from Practical Sailor.