The revitalising modulator was a medical machine invented by Professor James Zarn, his goal being to cure hangovers. He eventually died as a result of entering his modulator with a bottle of Voxnic. (PROSE: The Twin Dilemma)
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".
Azmael owned a modulator on Jaconda. The Doctor managed to reprogram it to work as a limited time machine. (TV: The Twin Dilemma)