Misleading title. Yes, the elusive ab definition isn't made in the gym so much as it's made in the kitchen. What we're talking about is getting strong abs. It used to be that if we tried to strengthen our abs, we'd lay on the floor. Most of us know that if you're laying on the floor, you're probably not a strong ab muscle (In case you don't know, the six-pack Abs are actually one muscle). Kettlebell-ISM could probably take a large amount of credit for getting us off the floor. While ab-training off the floor is a very good idea, I think that there's a strong possibility that the best way to get your abs strong is to reach up... and hang off something!
NO, NOT THAT WAY!
I mean your pull-up bar, or whatever you use for pull & chin work. Some of the highest ab activation exercises are actually the one's where you're hanging. As we've discussed in the past: properly done, the pull-up is an awesome ab exercise. You can make it even better by doing them with your legs in an L-sit.
For more direct ab work on the bar, then we could turn to the hanging leg raises (HLR). Most of us are familiar with this one...My reception to ab slings is kind of cold. Yeah, if you don't have the grip and arm strength to execute these then I understand. If you can do pull-ups without an issue, then you should have no issue grabbing the bar directly to do HLR. You'll save some money too because these things, like most strength training equipment, are ridiculously overpriced.
When I do HLR's, my preference is to touch my feet, or even my knees, to the bar. The important part of doing HLR's like this is descent: do so slowly, and under control. This isn't a speed-demon exercise. Control the movement! Besides, most of the good ab work is on the descent anyway. Rushing this risks hurting the lower back.
While you're up there, there's another ab exercise you could do called the windshield wiper. This one generally starts at the top of the hanging leg raise movement. From there, you move the legs in a semi-circle... like windshield wipers! The level of difficulty can be adjusted two ways: smaller circles or bent legs. Both make the movement easier. This is a nice and easy, controlled exercise too. Remember what I said about fighting gravity a while back? Well, gravity wants to pull your legs down. RESIST!
Then, there's always the possibility of combing hanging leg raises with windshield wipers. Start from a hanging position. Then, pick your knees up to the bar but don't lower yourself just yet. Now, do a windshield wiper. Bring your feet back to the middle and lower yourself.
You could throw this in after doing one, or more, pull-ups too. Start by doing the pull-up(s), then bring your feet up to the bar and then do a windshield wiper set, then slowly come back down. This combination is pure hell on your grip and your abs! Even 5 "reps" of this can be very brutal! It's kind of like burpees but for the bar.
Those of us who follow BW training know this guy, HIT Richards. While his insanely bad-ass training videos are well known, I wish that even more people saw what he's capable of. There'd be a whole lot less doubt about the legitimacy of BW training left to argue about. Much of his work revolves around pull-up bar work. Between his obvious power and his carved physique, he proves pretty decisively how well hanging exercises work the mid-section of the body. Even Pavel Tsatouline commented on the how well pull-up and chin-up work the abs: find a guy good at either with a weak rectus abdominis. It just goes to show how valuable this very simple set-up can be.