From the rulebook:
ARTICLE 18. HIP-DROP TACKLE. It is a foul if a player uses the following technique to bring a runner to the ground: (a) grabs the runner with one or both hands or wraps the runner with both arms; and (b) unweights himself by swiveling and dropping his hips and/or lower body, landing on and/or trapping the runner’s leg(s) at or below the knee.