The Left Twist stitch creates a mock cable, crossing one stitch over the one to its left.
Working with the next two stitches on your left-hand needle, knit into the back of the second stitch but leave it on the needle. Then bring your right-hand needle around in front of the left-hand needle and knit the first stitch on the left-hand needle as normal. Finally, slide both of the stitches you’ve been working with off the left-hand needle. Twist complete!
This video from New Stitch a Day (and the corresponding written directions) is quite clear:
You can also work it as a C2F, if that’s easier for you.