Emergency power booster unit

The  was a component on the TARDIS console. The Second Doctor once tried to find it after he noticed that the TARDIS was abnormally "using more power than [it was] storing". He tried to reassure Zoe that the condition could be corrected by simply using the booster unit. Eventually, he did find it and it did increase the power levels. However, it was also the last TARDIS control lever pulled before the TARDIS disassembled and the crew were pulled into the Land of Fiction. (TV: The Mind Robber)

Residual power stored within this component would later be used by the Sixth Doctor to affect an emergency flight to the planet Varos. He found it could be used to bypass the failing circuitry and perform one short materialisation without zeiton-7. (TV: Vengeance on Varos)

Behind the scenes
TV: The Mind Robber never makes particularly clear the booster unit's role in the apparent destruction of the TARDIS. The disassembly happens immediately after the Doctor throws the switch, but it's unclear whether it causes the TARDIS' woes.