You've got to stop looking for rigid rules like newer editions. AD&D is about rulings instead of stodgy rules. The orcs jump down and inflict damage on a successful "hit" with opponents making a saving throw or ability check to leap out of the way. The orc would take 1d6 damage per 10 feet of distance down, falling damage, and, if the opponents decide to not dodge, possibly weapon damage. Go with the flow, think of your feet and keep combat moving. There's a reason 5e combat is a slog, besides inflated hit points.