there are a few mathematical formulas that one can use to explain this, but the easyest way to explain this (imo) is, lets say you put a piece of paper on a table and press on the center of it with the eraser side of a pencil (vertically), and spin the paper with the pencil (the contact of the eraser with the paper is the clutch in this case). then replace the piece of paper with a book (-_-) and press down on it with the pencil (same force as on paper) and try to spin the book (applying the same torque as you did on the paper). obviously you "clutch" is gonna slip. and btw slipping is the only way that i am aware of that might damage a clutch (because of the engine torque).