>Deflect Missile (Recharge 5–6). Trigger: The giant is hit by a ranged attack roll and takes Bludgeoning, Piercing, or Slashing damage from it. Response: The giant reduces the damage it takes from the attack by 11 (1d10 + 6), and if that damage is reduced to 0, the giant can redirect some of the attack's force. Dexterity Saving Throw: DC 17, one creature the giant can see within 60 feet. Failure: 11 (1d10 + 6) Force damage.
Assume a Stone Giant Deflect Missiles a boulder- how do you explain the Force Damage narratively?