Janny Hu recently wrote an article about how Nelson is making players accountable for their defense.
What I like about this system is that it makes the players more aware their defense. When coaches start counting up how many times you screwed up, you definitely try harder not to be the guy with the most tallies. I bet Nellie will have a bulletin board ranking each player. That's really embarassing if you're always at the bottom. Accountability is the key here.
I'm sure the coaches also break it down versus team and versus a specific opponent player. Furthermore, I can see them looking at the marks by quarter. So if you play better D in the 4th quarter, you'll probably get serious crunch time minutes.
Of those players getting major minutes, who's going to get the most negative marks? And the least negative marks?