Turn the yellow knob to get 3 working states.
1. The red arrow points to the left: the black pin on the knob prevents the violet shaft from contact with the disk so the latter can rotate free.
2. The red arrow points up: the black pin does not contact the violet shaft. The latter contacts the disk and brakes it with a sufficent friction force caused by red spring.
3. The red arrow points to the right: the black pin pushes the shaft towards the disk with large force so the disk can not rotate.
It is possible to make teeth on the outside cylindrical surface of the disk and the shaft end in shape of a pawl.
A paper scroller consists of two such mechanisms. By combination of their working states people can scroll paper fast, slowly for searching informations or keep it immobile for reading.
This mechanism was made on request of a YouTube viewer from Israel.