Sure. It's when you book one night (the first or second night of your vacation, depending on if you want free parking for one day or two) at the cheapest possible WDW onsite hotel. Set up an account through My Disney Experience, link your reservation and your tickets and you can make FP+ for the length of your tickets.
You can make FP+ 60 days before your "throwaway room" check-in date.
I'm staying at Wyndham BC next month for 10 days beginning 04/23, with 7 day park tickets. I booked a campground for 04/23 for $90 (including all taxes) that I have no intention of using. I am going to check-in so I get free parking on the 24th when we drive to the MK, but you don't have to check in if you don't have a car.
I was able to book FP+ for 7 days (I have 7 day tickets) over my 10 day vacation. If I had 5 day tix, I could book Fp+ for 5 days, if I had 10 day tix I could book for 10 days.
Make sense?
eta - I have no idea how the DTD hotels work. If you can book FP+ if you stay there, then you can book one night there as your throwaway room. I know you can't with Swan/Dolphin.