As the ninth Doctor sucked the vortex energy from Rose's mind and body, he knew that he would die. Yet the little smirk betrayed no fear. "Timelords have this little trick." He blithely told his companion. "It's kind of a way of cheating death."
Rose may have been stunned by proceedings, but the viewers watching at home were not. We'd seen it nearly a dozen times before.
The actual mode of regeneration changed with the advancing special effects, but we knew what the Doctor knew. He was about to transform every cell in his body, and emerge in a totally different body.
This was the trick of the Timelords of Gallifrey. Not peculiar to the Doctor alone, we'd seen it happen with the Master, Romana and River Song too.
At a moment of mortal danger, a process would begin, which could not be stopped. The old, dying body would disintegrate into the ether, while the new would form without the injuries, poison, age or whatever other ailments had killed the former.
There were variations on a theme. The second Doctor was forced into his regeneration. His whole body appeared to spin in space. The fourth Doctor seemed to know and welcome his Fate. He beckoned for the assistance of a mysterious figure called The Watcher, who apparently delivered the Timelord's next form. It was all very calm and sedate. The tenth Doctor raged against the coming of the night. He didn't want to go and his regeneration was so explosive that it set the TARDIS alight.
Fans of the show would be treated to the spectacle of a new actor, prancing around in the familiar costume of the last. The hapless companions would each have to discover the 'little trick' for themselves.