Bloodfasting, or simply fasting was a process by which two individuals (at least one of them a vampire) could become bonded, described by the Doctor as "a psychic link between two vampires".
Bloodfasting was enacted through both individuals drinking a small amount of each other's blood in quick succession. The amounts of blood were not necessarily equal; per Joanna Harris's calculations, a successful fasting between her and the Eighth Doctor would take his drinking five milliliters of her blood, while she drank ten of his.
The bloodfasting was a highly traumatizing process, which would leave the participants haggard and disoriented for quite some time after it was complete. A mortal participant's skin would also become cold to the touch for a time after the fasting. For these reasons, it was easy to confuse bloodfasting with the beginning of a vampiric transformation.
As a token of trust, the bloodfasting would ensure that one member of the Bloodfasting would become vulnerable to the other's injuries; if one was killed or wounded, the other would suffer the same fate at the exact same moment. This ensured that neither participant would try to murder the other.
The bloodfasting also granted a measure of telepathy between the two participants, which was at its strongest while the change began. During this phase, each could feel the other's thoughts and emotions and get glimpses of their memories. This effect receded later on, with the two linked individuals only getting subconscious feelings and the occasional "ghost image" from each other.
To ensure mutual trust after the Eighth Doctor made a peace offering to her coven of vampires, Joanna Harris enacted a bloodfasting between the two of them, proving that it could be done not just between two vampires, but between a vampire and a Time Lord. (PROSE: Vampire Science)