Source: Warhammer Armies Project: Unofficial 9th Edition

Drop Rocks
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Once per game, during the Remaining Moves sub-phase, models with this special rule may drop rocks unless they are fleeing or have declared a charge that turn. All models in a unit must drop rocks at the same time. To drop rocks, select one unengaged enemy unit that the Terradons moved over in that turn. That target immediately suffers D3 Strength 4 hits for each Terradon in the unit, distributed as for shooting attacks.