Refers to natural torque in regular twill fabrics caused by the geometry of the weave such that the fabric shrinks more perpendicular to the twill line than along the twill line.
Although the fabric is woven with the warp and weft at right angles, the twill line will cause the angle to torque upon about 50 washings. Denim is purposely skewed or biased to that degree in the same direction as the twill line to relieve the natural torque in the fabric and prevent the side seam from twisting around to the front of the lean.
Source: Tellason Denim Terminology