Temporal gate

A temporal gate or "gateway" was means of projecting something a second into the future, thereby making it undetectable in normal space and time. Passing through a gate unprotected could age people and drones.

The Nine kept his prison secured by a temporal gate, enabling him to hide the Doctor's companions so that even he couldn't rescue them. As she was a complex temporal event, Charlotte Pollard was able to pass through the gateway and help the others escape. (AUDIO: Companion Piece)