I didn't get it either to start with. Just think of the ROM as a list of numbers printed out on a piece of paper, and the FPLA as a sheet of acetate with a few numbers printed here and there. Then put the acetate on top of the paper. When you look at the whole, you see mostly the original ROM contents, but when you look at specific places, the values are "overwritten" by the FPLA. Clever stuff, really. You're not likely to be implementing a lot of extra features in 48 bytes, but it should be enough to add a few tweaks or bug fixes in the field. The hardware "challenge", if you like, is catching a few random addresses and slipping a different value onto the data bus, whilst disabling the original ROM for those particular addresses.
Ahh, I see. Now I DO get it. Yes, very nifty.