See that guy in the middle, casually checking out Lindsey Vonn as she CRUSHES this insanely challenging ab exercise? If it were me, I'd be staring wide-eyed with my mouth open, chin on the floor. I'd be like, "Hey Lindsey. Holy sh*t! Can you show me how I can be as incredibly badass as you? Thanks."
This move is a double dose of muscle-building awesomeness. First of all, Lindsey is hanging from a bar and holding her body in a sort of half pull-up with her elbows bent to work her upper body. And then she adds in the rotating knees to chest to massively target her core.
I know this move looks difficult, but you can modify it by just hanging from a bar with your arms straight. If moving your knees from side to side is too hard, just practice raising and lowering your knees. If hanging from a bar is too much of a challenge, just do a Seated Russian Twist instead — it'll still work your abs!