Defaults: DX-3 or Throwing-4
Dropping (DX/Average) is the skill dropping heavy objects on your foes while flying.
B189.
B241 under Dissipating Held Melee and Missile Spells mentions that dropping spells to hit something below you is a Free action
B364 reiterates that.
Catch Spell also specifies THROWN attacks, so it might not work against attacks dropped on someone.
B400 applies:
- If a random attack comes from directly above, treat “feet” as “hands” and “legs” as “arms.”
B431's Damage from Falling Objects explains the damage dropped items do.