On unertl scopes, the whole scope tube moves forward about an inch every time you fire it. Early Unertls had no spring and the shooter had to grasp the scope and pull it back into position after each shot. Later the spring was added and a light film of oil was left on the scope where it rides in the front ring (where it is tensioned with a spring-loaded plunger). The spring made the scope rebound back into position so the shooter did not have to do it manually.
All this was necessary because the Unertl scopes with their length and mass would often wallow out the dovetail mounts after a few hundred rounds and lose zero. The moving tube concept allowed that inertial energy to be safely dissipated.
Hope that helps.