Any unit that is at least ten models wide is counted as being a horde. Warriors in a horde can make supporting attacks from the third rank, not just from the second!
This represents the crush of bodies driving yet more warriors into striking distance, as well as the back ranks surging forwards to assail the unengaged enemy warriors. If the unit drops below the minimum frontage it ceases to be a horde and loses the ability to make these extra supporting attacks.
