Body-print

Body-print technology was employed by the Zygons to assist their limited natural shapeshifting abilities.

It was essentially a biotechnology which acted as a bridge between a given Zygon and a life-form they wished to emulate, and literally imprinted the original onto the copy. The "originals" would be kept in stasis, attached to a machine. A given Zygon could then access the machine remotely and request the shape of any life-forms then hooked up to it. (DW: Terror of the Zygons, NSA: Sting of the Zygons, DWY: "Rest and Re-Creation"). When killed, a Zygon would lose its link to the body-print machine and thus revert to Zygon form. (DWM: "Skywatch-7")

If the body-print machine itself were damaged or destroyed, Zygons could shapeshift, but only in a limited fashion. Using solely their natural abilities, they could maintain a given form for up to two decades, but they lost the ability to transform into anything else, or to revert back to Zygon form. (BFA: The Zygon Who Fell to Earth)