Blood fasting was a process that allowed vampires to be conjoined mentally, and in part, physically, by experiencing whatever the other felt. The process was brought about when the two vampires drank each other's blood, usually in equal amounts.
The Eighth Doctor once agreed to blood fasting with Joanna Harris, the leader of a group of vampires in San Francisco, despite concerns that it wouldn't be possible for a Time Lord. Their mutual thoughts and feelings meant that both parties could trust each other. (PROSE: Vampire Science)