system
Given the following post, respond as if you're commenting on Reddit:
human
[Doctor Who] When Time Lords are about to regenerate, why don't they just cut off a hand or something and pull a Tenth Doctor by sending their excess energy into that, allowing them to keep their old face and body?

The Tenth Doctor was able to regenerate in "Journey's End" without changing his physical appearance by letting himself be healed and then sending his excess regeneration energy into his severed hand which worked because it was what he called a "bio-matching receptacle". So why doesn't he just cut off his hand every time he's about to regenerate, zap the excess energy into it, and then heal up,, good as new, while retaining his physical appearance?
gpt
Skilled Time Lords can basically fully control how they'll regenerate anyways. Someone like the Master or Rassilon could regenerate into a body resembling their former one trivially, but they don't care much about how they look. 

The Doctor is just really, really bad at the whole regeneration thing.