How does bitlocker work? ( low level fread/fwrite )

[Fadey]

I've noticed that bitlocker is doing real time de/encryption. By that I mean instead of decrypting the whole drive, it decrypts only the requested part or buffer from drive what program or os requests at the time.
Decrpytion seems to be happening between when data is load from drive onto ram and before the program receives.

If bitlockers seems to be able to do it, could I make my program do it?
Change the behavior of fread/fwrite os wide, get 'between' and perhaps change the buffer before it gets to a programs who ever fires fwrite/fread.

How could I?