Time rotor
From Tardis Wiki, the free Doctor Who reference
The Time rotor was the central column in the center of the Tardis console. When the Tardis is traveling the rotor rises up and down until it has reached a destination. The aesthetic design, along with the rest of the Tardis, has changed throughout the Doctor Who franchise because of the different TARDIS 'themes'.
It is interesting to note that in the Dalek movie spin-offs, Doctor Who and the Daleks and Daleks - Invasion Earth 2150 AD, the Tardis lacked the time rotor or even a central console.
It rises up and down during dematerialisation from the universe(s) into the Time Vortex, while flying in the Time Vortex, and materialisation into universe(s).
