In RoP: M, the Temporary Might inferiority says that "...like other magic characters it can still regain Might Points immediately by consuming vis...". This is the only reference to such an ability that I can find in the book, or anywhere else for that matter: Is it expounded upon elsewhere, or is this the only mention?
The bit you're looking for is on page 29, in the third column. This section tells all the innate advantages and disadvantages of a magic character, including how they can consume vis to regain Might.
Thanks. No idea how I missed that. 