The Swirl was a form of regeneration used by Time Lords under attack. It involved a speedy and efficient face swirl. (PROSE: How to be a Time Lord)
An example of this was the Sixth Doctor's regeneration, which occurred as the TARDIS was forced down to the planet Lakertya by the First Rani. (TV: Time and the Rani) By one account, the regeneration occurred as a result of the TARDIS being struck by beams of radiation deadly to Time Lords. (AUDIO: The Brink of Death)