They took the primal energies beamed from both the original Eye of Harmony on Gallifrey and generated by the perfect block-transferred copy of the Eye at the heart of each and every TARDIS, and converted it into the titanic energies necessary to propel a TARDIS through time and space and maintain the machine's dimensionally transcendental interior. It could be considered that if the power room was the circulatory system and the telepathic computer was the brain, then the generators were the heart of the TARDIS.
The door to the dynamorphic generators was five metres high, embossed with the swirling pattern of the Omniscate, the seal of the Time Lords. The room that housed the generators was at least a mile high, stretching off into the distance in all directions, without walls, with the door existing as a freestanding square of gunmetal grey. The generators themselves were millions of trachoid time crystals, thick pillars of emerald crystal reaching upwards from a stone floor, the interior flickering with tiny stars of golden light flowing towards the distant ceiling. In the Master's TARDIS, the pillars vanished into dark thunderhead clouds, where thunder boomed and lighting glittered off the pillars every few seconds. (PROSE: The Quantum Archangel)
The dimensional collapse caused by the dissolving matrix of block-transfer computations that comprised Castrovalva had caused serious damage to the Master's's generators, disrupting the link between his TARDIS and the Eye of Harmony. (PROSE: The Quantum Archangel) This caused him to find a new potential power source in the Xeraphin gestalt, the Fifth Doctor theorising that he must have exhausted his dynamorphic generators. (TV: Time-Flight) The Master later modified his generators to draw power not only from any version of the Eye of Harmony, but enabling them to tap into any power source in the immediate vicinity.