Abstract: A fluid pressure spring (8), such as for opening the hatchback door of a vehicle, comprises a hollow cylinder (10) having a fixture (24) for pivotal connection to the hatchback door. A piston (30) is connected to a hollow piston rod (114) which carries a fixture (46) for pivotal connection to the vehicle body. The interior (40, 42) of the cylinder is filled with gas under pressure, and the gas extends into a chamber (54) within the piston rod via a bore (62). A free piston (50) within the hollow piston rod (14) is subjected to exterally applied hydraulic pressure via a pipe (20) connected to a second chamber (56) within the hollow piston rod (14). Controllable increase of this pressure moves the free piston (50) to augment the gas pressure within the spring and to increase its lifting force. In this way, a remote control can bused to unlatch the hatchback door and temporarily augment the gas pressure within the spring to cause the door automatically to lift from the fully closed position to the open position.