The constant shifting of the weight makes it even more amazing as you have no choice but to keep control of the bell or else you might hear something crash unless you're outside and you hit a window or a fence. When it comes to weight; I believe to develop great mastery and full flexibility and strength in the exercise itself, be sure to take on no more than 20 pounds, that may not sound like a lot but because of the control and accurate amount of tension it's going to feel way different. I have heard of the 40 lb. 100 rep challenge and for the most part it's basically a marketing ploy. I'm going to give you a quote I made on a forum where it'll give you a complete outlook on why the weight doesn't make you the man, but the consistency:
"Its not the weight that makes you a man, its the consistency to train and learn the value of your body's capabilities according to your range of motion, awareness & the will to stay in peak condition without risking injury. More often than not, ego breaks more than the body does. The physical injuries are just icing on the cake because people buy into the fact that if they don't train beyond their natural abilities they're not worth a damn. I've pushed myself hard for more than a decade and only got one small minor injury during that entire period."
Read that with complete mindfulness. Don't buy into the notion that bigger is always better, you don't need to go to extreme reps with a lot of weight to get amazing results. Pace yourself little by little down to the smallest fraction and you'll still get results. I'm a big dude and never needed to move a ton of weight to become strong, the things I have done were carried over because of my training. Trust me when I say this, no matter how big or small you are, you can get crazy strong with minimal equipment and no more than 20-25 lbs. worth of actual weight.
Be in control of your exercise or it will hurt you more than just physically. When it comes down to it, if you want powerful Tarzan-like abs, this is a top exercise to make that a reality. Practice with focus, intention, muscular tension (Muscle Control) and awareness of how your body moves within the exercise. Make it work for you and if you want to increase weight, do it for progressive reasons, not to prove how strong you are. Train smart and train awesome.