Broken twill is a weave variation that periodically reverses the direction of the diagonal line instead of running it consistently one way, as regular right hand or left hand twill does. The result is a fabric with a less pronounced, sometimes almost herringbone like diagonal pattern.
The main practical benefit is that broken twill denim resists the tendency of standard twill jeans to twist so that the outer seam rotates slightly toward the front of the leg after repeated washing. Because the diagonal direction alternates, the forces that cause twisting largely cancel out, keeping the side seams straighter over the life of the jeans.
Broken twill became well known through a small number of denim brands built specifically around solving the twisting problem, and it remains a useful detail to look for if you have had jeans twist on you in the past. See our jeans washes and finishes guide and the entry on twill weave for related background.
