TWRP 2.2.0 here.
I tried 2 nandroids in CWM 6 back to back just to confirm in my head how it was working.
I think the nandroids are doing a dif check between old/new files and if file remains the same it makes a reference to said file.
If file is changed it backs it up.
Cool...because the second nandroid took literally 15 seconds.
Meaning it ran a diff and said "lul all this is the same. I'll reference to the backed files again. Here's nandroid#2."
(If my thinking is correct, anyway).
Never flashed anything with it.
I personally just prefer TWRP.