What I hate about doing Core exercises is the Sore afterwards (!) I took one class on Wednesday night and I can still feel my "effort" today, three days later; i.e. can't laugh or sneeze without feeling the muscle pain.
Building up the Core (which is kinda like a thick belt that wraps around the abs and around the back) is vital in a yoga practice not only to give practitioners (inner) power -- there are so many yoga poses that require Core strength such as Warrior 3 (Virabhadrasana III) -- but to protect our lower back. Having Core strength is also vital for the inversion poses.
There are many ways to train the Core ... and one of my old yoga instructors, Lawrence, used to ask us to get into various poses that require Core strength, which I enjoyed. The practice also helped us focus. There is also what I'd called the drill method, which basically means you spend the entire class just targetting the core with repetitive exercises (a bit like crunching in fitness training... that was what the Perfect Human Specimen used to do during part of his Core classes) ... last Wednesday our instructor adopted the latter method for the ENTIRE class. I mean, that was kinda sadistic (but I think most of the (female) students actually enjoyed it despite all the grunting and moaning ... could be the myth of weight loss at work again).
I don't think I will go to his class again (esp I tend to laugh and sneeze a lot). Am resting this weekend but will probably get back into some serious practice next week....