A simple rule, a fellow teammate cannot help advance a ball carrier through contact with said ball carrier. The team can block all they want, but no players can push the player forward. Conversely, on D a player cannot forcibly contact a ball carrier who is in the grasp of a tackler. He can grab ahold, but no cleaning the pile, or laying wood to an upright, but being tackled player. I saw one today, where a WR was stood up, and a LB layed the wood to him while he was being firmly held while being tackled. Assist a tackle, but not cleaning his clock.