HPLRP is a little funky, because it is not actually obligating.
Here's how it works:
Once you're board certified, you can tell the Guard "Hey, I want HPLRP!" They give you the paperwork and you sign it. One year later, $40K is paid to your federal loans. It is NOT obligating. Meaning, if six months from signature you say, "Nah, never mind..." then you can still leave the Guard, but you don't get the money paid off your loans? Make sense?
The only exception to board certified folks is if you're in residency AND taking STRAP. If that's the case, then in R3 or above, you can take HPLRP. So if you did a General Surgery residency, in R3, R4, and R5, you could get HPLRP $40K each of those years. This does not add any time to the obligation you'd owe for STRAP (which if you did for all of residency, you'd owe 10 years drilling). You can also continue with HPLRP for three more years right after residency, if you wanted, for another $120K total, but those three years that you took HPLRP post-residency would not count towards your 10 year STRAP payback.
All clear?