Usally while the ball carrier was on the ground. Subtle, but intentional things like rubbing the forearm through to the face/neck while "attempting" to get off the pile or stand up.
It seems to be an MO for that defense. That old forearm just seems to "somehow" get under the chin strap or on the neck during a tackle, and particularly the follow through of a tackle. It's just dirty play, plain and simple.