The knee drop is a good tool if your opponent grabs your leg and attempts to drag you down. Striking in this instance is much more stable and feasible than kicking or stomping, which puts you on one foot with a very high center of gravity. Note the technique I'm thinking of involves pivoting the foot and swiveling the hips like throwing a cross as you drop to one knee. These can defintely break ribs and cause all sorts of problems for your opponent. I'm not advocating the jumping and dropping down with both knees like some kind of WWE top rope craziness.