The purpose of the BFD in an HT is to EQ your sub. Basically, you sweep the room with an SPL meter and tone generator, and build a chart of your room response. I'll take measurements from say 20 hz to 100 hz. Any room will have peaks (read - loud, boomy bass) and dips. You can use the BFD to tame the peaks. Can't do much about the dips, other than move the sub, move furniture, treat the room etc. It will give you a much better bass response in the end (hopefully).
You essentially EQ or cut out very narrow (in Hz) bands (the peaks) of bass. It is similar to how I edit a .wav file after I've transferred an LP to my hard drive. If there are any pops or clicks, I can isolate just that "pop" by expanding out the wavelength. I then edit/cut out maybe one tenth or one hundreth of a second of the song containing the "pop". You'll never miss the tiny amount cut out of the song, but the pop will be gone.
JD