So a theory on Matt Smith's regeneration.
That MASSIVE explosion was the Doctor regenerating from his first FULL cycle of regenerations (1-12, 13 incarnations total). Then the ACTUAL change from Matt to Peter was that quick little one. If everyone will recall correctly, the Doctor's original regenerations were very subtle (due to primitive special effects, etc). But as time went on, his regenerations became increasingly violent (6's was a rainbow explosion, 7th involved electricity sparking everywhere, and John Hurt to Matt Smith's was a golden fiery expulsion of energy). Until David Tennant's second regeneration which SHOULD have been his last. So the consequence of a brand new regeneration cycle meant that he had to regenerate FROM that original cycle into the new one. The whole new cycle of regenerations will all start subtle, and then become more violent as time goes on.
Now, in terms of Regeneration itself, it's not that complex. Time Lords have 12 regenerations, which means 13 lives in total. (I mean, they have to be born first [or Loomed first, if you believe that story]).
Picture chart for reference: i.imgur.com/3Dxax3I.jpg