I presume he means doing sit ups with a weight plate held accross your chest which I have seen done but never tried it myself. I find the best thing for abs work is just lots of different sit ups (half sit ups, twists) and crunches. Leg raises also really work the stomach muscles well, if you find normal leg raises easy do it with ankle weights on. I have been using a 7kg medicine ball as well and it has worked like a dream, ive been using it for about 4 months now and my abs are looking better all the time. Some people dont agree with the medicine ball though as they say it causes too much internal damage, it is painfull but I have found it works brilliant and I can take shots so much easier in sparring.