Grabbing the Helmets??

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"and as the ball was snapped, they would grab the offensive linemen by the back of the helmets and shove their heads down..."

9-4-2(h) - Illegal Personal Contact (Facemasking)
No player shall grasp an opponent's face mask or any edge of a helmet opening.


By the sounds of it, the d-lineman is grabbing the back of the helmet which entails a facemask call. Since it's intentional, that's a 15 yard penalty.

Granted, this is by your description only. I would have to see the play to determine whether he was grabbing the back of the helmet, or simply pushing the o-lineman's head down as the o-lineman attempted to cut the d-lineman.