This signal is used by the Referee usually to indicate that illegal assistance has been given to a runner by an on-field team-mate - eg a strong push or the ball-carrier grabbing a team mate's belt and being "towed". This is a comparatively rare foul.
Penalty5 yards from the spot of the foul.
Begin the motion with both your arms held at your side with the palms facing forward, and move them forwards in a gentle pushing motion. It is not necessary, and indeed undesirable, to push the hips forward at the same time.
Finally point to the offending team by stretching an arm out from the shoulder to the horizontal.