An engine driven lockup will increase force directly proportional to the engine speed. An input shaft driven lockup will not apply any force until the static clutch plate force is enough to rotate the input shaft and generate shaft speed (wheel speed).
If you are too light on an input shaft lockup, it will fry the clutch almost immediately. On an engine driven lockup, the engine will increase revs and (usually) the clutch will still add enough force to move teh bike and prevent the clutches from frying.