Winder for loop end mainsprings, to be used with the mainspring clamps. This is to take a unwound loop end mainspring and wind it up to get a clamp on. When the C clamp is on, it is much easier to manage in the clock movement assembly process. There is no 'safe' operation on this one. It is both dangerous and tricky. You would use the great wheel, put it through the hole in the device, clamp it down with something under the clamp to protect the teeth of the wheel. Put the mainspring on it and the loop over the arbor of the device. Use a mainspring key to wind the mainspring up on the great wheel that is clamped, when the mainspring is small enough put the mainspring clamp on it. Now the mainspring is wound and easier to put into the disassembled movement to reassemble again.