a player in motion:
1. Can block low if the block is from "in front" or between 10-2 (rule:9-1-6-a-1 & 2)
2. However he can NOT block low back toward his GL once the ball leaves the zone (9-1-6-a-3)
3. And can NOT block low back towards orginal postion of ball at snap 9-1-6-a-4