What happens is the belt has grooves in it that match the shaft on the primary clutch. When the belt gets worn, it can move over one groove which causes the belt to hold pressure on one side of the primary so it never disengages. Sometimes, with the cover off and the engine not running, you can move the belt back to center, start the machine, and then get it in gear normally. But once you rev the engine to engage the clutch, the belt will move again. I don't recommend trying to run the machine without the cover, very dangerous.