0% understand it, because it is beyond comprehension. It makes no sense whatsoever. It is like asking "who understands the concept of 2 + 2 = yellow?"
If you hear a JW express confusion/lack of understanding of how you can scripturally support multiple generations actually being 1 (overlapping) generation, you could always say:
"It's really quite simple...although they are seperate generations from a human perspective, from a Bible perspective, they are 1. Sort of like when Jesus said: 'I and the Father are one.'...seperate from a human perspective, but a single being from...oh wait, that's the Trinity! Nevermind! The using that logic to defend the Trinity Doctrine never made any sense!"