Damn, I love this crunch. I don't know how it escaped my attention until now. Usually we just isolate the rectus abdominus muscle with regular crunches, or the obliques with side crunches. This one hits both in a fun way.
Lie on your back, hands lightly touching the ears. Lift the shoulders slightly and instead of crunching up, rotate the torso to the left and up in a circular shape. Go all the way around until you're down on the opposite side. From here you can keep going in the same direction for a few more reps, or, stop and go in the opposite direction, which is pretty hard.
One of the hardest things about abs work is keeping it fresh. Not just to alleviate boredom, but to find new ways to stimulate those muscles that get too good at the usual exercises.