Ghost Shift was the name given to the phase when ghosts appeared.
Due to the opening of the Void, ghosts began to appear all around the world. Torchwood One built a machine to turn on and off the ghosts' appearances. The phase when the ghosts were present was given the name Ghost Shift. The public were given set times when the ghosts were going to appear. There were three shifts a day: morning, mid-day and afternoon, with mid-day being the shortest as it only lasted two minutes.
The Ghost Shift was turned on and off using levers in Torchwood Tower. When active the hole in the Void glowed with a white light. Cybermen overrode the ghost shift, activating it. More ghosts then ever before came through and were revealed to be Cybermen. When the void was closed the ghost shifts ended. (TV: Army of Ghosts)