Doctor Who: A Regeneration FAQ
1. Developed by Cardinal Rassilon, Gallifreyans were implanted with pockets of self-replicating, biogenic molecules that could be released to renew dead and dying cells. This process is activated by a hormone called lindos that triggers the process.
2. A lesser-known theory is that Rassilon actually managed to turn Gallifreyans from a double helix race into a triple helix race, with the third helix holding the secret of regeneration, whatever it may be. This could explain how the ability is passed on through genetic lines.
3. Time Lord regeneration has something to do with their constant exposure to the Untempered Schism, a gap in the fabric of reality that the time vortex can be viewed through. Time Lords are required to gaze into the schism upon graduation. This theory would partially explain the semi-Time Lord nature of River Song, who was conceived in the time vortex.
Do Time Lords always regenerate as the same sex/race/species: We know that significant skin color changes are possible from the regeneration of Mels to River Song, and even more striking colors like blue from Romana. However, in general a Time Lord tends to remain close to their original form in terms of gender, skin color, and accent.
To what degree Time Lords can control what they become is open for debate. Romana changed form several times attempting to find one that suited her, and River Song stated she was concentrating on a dress size as she regenerated from a gunshot wound.
Side note... never really noticed how sexist that came across before.
Regardless, we know gender changes are possible because The Doctor mentioned they were in the case of his friend The Corsair. One theory was that suicide would instigate a gender swap, but as many of The Doctor's deaths could be considered suicides this is likely not the case. Other more extreme changes or deformities have been hinted at in various apocrypha, but have not appeared in the series proper.
Does it hurt?: No two Time Lords, indeed no two incarnations of the same Time Lord, seem to react the same way during or post regeneration. We've seen The Doctor rise more or less ready for action after some moments of slight dizziness, and we've seen him need significant convalescence after rebirth. Severity of the inducing cause of death does not seem to be a significant factor in how difficult returning to normal function is. Of all the mysteries of regeneration, the birth pains of the new Time Lord incarnation are possibly the least understood.
Piece concludes on next page.