Well, I'm not too sure myself since I have not really gone to that part of the code, but this is my (shallow?) understanding.
Cutting and pasting files within the same drive is a MFT/FAT reallocation; upon moving the file from one place to another on the same volume, the file reference is modified. Moving a file between volumes however involves the deletion of the file from the source drive and the creation of the exact file on the destination drive. This will leave the contents of the old file on the old drive without ofuscation and can therefore be recovered.
I believe Secure Move does the move operation then erases the file.