I've actually felt as though my core has received the most beating when using the ab crunch MACHINE. The only you sit in, adjust resistance weight, hold handles (usually above shoulders), and crunch. I've felt as though it wasn't so much targeting my abs when doing it with "too much" resistance - ie if I can set resistance to 40 and get 10 reps to feel burn in my abs, I'd set resistance to 55 and push for 8 reps. Reps done with too much resistance seem to lift your ass off the seat lol....
But yeah, doing this method really hammered core muscles more than abs...I know it's not a traditional or even logical way of doing it, but I can assure for myself at least, it works. Makes me feel like I'm going to shit my pants on the last few reps LOL