erm, he didn't mention copying cds, on-the-fly has nothing to do with .cue and .bin files.
You already have the data on your hard drive, on-the-fly means when you are duplicating a cd, not when burning an image file, and refers to skipping making an image file and copy straight from one disc to another using ram and buffers. It is not recommended, but that's for copying cds not what izac asked