The model part gains Grind Attack) (X).
After each Round of Combat, X increases by~1 (e.g. Grind Attack) (1) becomes Grind Attack) (2))
When the model is not Engaged in Combat, X is set to~1.
When a model part with is in the second rank, and not in contact with any enemy model, it can make all its hits from Grind Attack) across models directly in front of it in the Front Rank.