The 1403 has electrically-controlled valves to send fluid from the constantly-running pumps to the hydraulic motor to do a space (line feed) or skip (form feed). There's a back pressure bypass valve that relieves the pressure when the printer advance stops.

The 1403 Field Engineering manual goes into great detail on the hydraulic system, starting at page 56. The momentum when stopping is described on page 62. Link: http://ibm-1401.info/0KenShirriff/IBM_Field_Engineering_Manu...