English: APAS-75 docking unit. Unlike previous docking systems, both units could assume the active or passive roles as required. For docking, the spade-shaped guides of the extended active unit (right) and the retracted passive unit (left) interacted for gross alignment. The ring holding the guides shifted to align the active unit latches with the passive unit catches. After these caught, shock absorbers dissipated residual impact energy in the American unit; mechanical attenuators served the same function on the Soviet side. The active unit then retracted to bring the docking collars together. Guides and sockets in the docking collars completed alignment. Four spring push rods drove the spacecraft apart at undocking. The passive craft could play a modified active role in undocking if the active craft could not complete the standard undocking procedure. Pyrotechnic bolts provided backup.