Gotta concentrate on the area you are trying to work. Gotta know the breakdown of the exercise and concentrate on doing it correctly. If you are doing certain exercises like these to work the abs or core or trunk or whatever you call it then you will want to really concentrate on feeling the rotation in your stomach area (at least in that top picture).

I can kind of see how an inexperienced person might try just using their arms in the top picture. In the picture of the guy on the ball it's basically impossible to not be using your "core" and also legs/glutes to hold up your legs. The arms aren't gonna have much to do with it. I've never done that exact exercise but can just see that. I suppose that's a reverse hyperextension?

Hope I used the word "core" correctly. I usually just say abs as a general term I guess.

I've done exercises very similar to that chek wood chop. I was doing those a long time ago. A bodybuilding guy taught it to me. hahaha.. yeah true story. Guess I'm ahead of my time and apparently he is way ahead of all of you. Little sarcasm about the bodybuilding thing but true.