As far as I know SPR is coming from the book Professional No-Limit Holdem, which is a relatively new book from Matt Flynn, Sunny Mehta and maybe others, I don't know for sure.
In the FTR "Tools of poker" section you can find a bit more information on it, though not very much as the book hasn't been discussed much (as far as I know).

I haven't read it myself and am quite anchious to hear what guys like Fnord and Sauce think about it. One of the concepts in the book talks about something called SPR, which in my experience has something to do with pot/stack ratio for certain type of hands, pot comitting and related stuff.
But as I said earlier, I haven't read the book myself, only a few words about it. So someone that has read it can comment far better on the subject then I can.

Though I have to only have read a few words about it