They explain it like this :
Grades per play can be simpler to positive, negative, neutral. If you get a 0 (neutral) grade on any play then it means you do your job to a stalemate. Think offensive line. If the linemen clears a crater for the running back it's a positive play. If he's blown up by the defender it's negative. If the linemen does enough but didn't really move his defender, it's neutral. That's why their grades are very different than our eyes. We see a lineman not give up a pressure and we say "oh that's good" but they say "that's neutral". Flawed but I do like the idea.
