What does Anti-Twist mean?
Prior to raw denim undergoing sanforization in the finishing process, it’s natural tendency to twist in the direction of the diagonal twill weave is corrected.
Heddels explains Anti-Twist
Denim’s pattern is known as a twill weave, in which blue and white cotton yarns are interwoven. As denim was cut along the selvedge and woven diagonally and (traditionally) were unsanforized (not pre-shrunk), the legs would twist substantially when washed. The resulting effect would be an angled seam being shifted to the front of the pants.
Thus, raw denim is anti-twisted (or skewed) to prevent this effect.
An example of intentionally twisted G-Star raw denim (source: tradepar.co.uk)