The revitalising modulator was a medical machine owned by the Time Lord Azmael on the planet Jaconda, and used by him to "revitalise".
In the words of the Sixth Doctor, who claimed to be familiar with this kind of technology, it worked by breaking down the molecular structure of a being and putting it back together again; he defined the process "most refreshing".
The Doctor managed to reprogram the modulator to work as a limited time machine. (TV: The Twin Dilemma)