Generally, I don't like hype since I think most of it is bullshit and I'm not into that. I want hard facts and I don't really care if they're new or 100 years old. Still, I picked some different calisthenics for my abs that I don't normally do and combined them and I was stunned by the intensity of the workout and the results that I got. Here was the routine that I did:
1. Grasshoppers, 30-40 reps
2. Hanging leg raises, 8-10 reps
3. Mountain climbers, 30-40 reps.
4. McSweeney's ab contraction, 5 reps max tension
5. Powerflex Isos and/or classic isos
6. straight bridge, held for ten deep breaths
I repeated this whole set two or three times.
If you think about it, most of our abdominal exercises are variations of two calisthenics: The sit up and the leg raise. While there isn't anything really wrong with these exercises and their variants, they only work the abdominals in an up and down motion. You can vary them to get the obliques but still, it's an up and down movement. What I realized is that this misses one of the fundamental rules to exercising in the bodyweight world: working the muscles from all angles and directions. By confining myself to this one direction I wasn't training my muscles to their peak capability.
Plus, this side to side movement helps hit the troublesome obliques far better than trying to modify the leg raise and sit-up motion. Still, the side-side motion of the mountain climbers and the grasshoppers hits the abdominals in a new, and subsequently more difficult, manner. The straight bridge and the hanging leg raises demand much more out the obliques in order to stabilize the body during the workout.
Give this some thought and try seek out some ab exercises that have a side-side motion. You'll be surprised at the results.