Cryocoolers, including coolers, heaters, and heat pumps each having a first stage and a second stage, are modified with controlled and variable gas phase shifting devices for controlling gas pressure and mass flow between volumes of the first and second stages for improving the efficiency and temperature range of expander cryocoolers such as displacer cryocoolers using controlled valves as flow impedance devices and pulse tube cryocoolers using inertance gaps as flow inertia devices, for maximizing cooling between the first and second stages.