Singles: Yawn into random mon, check Y-info screen, mon falls asleep
Yawn against Dynamax, wait 20 seconds, switch out Dynamaxed Pokémon
Yawn vs. Safeguard
Yawn, then Safeguard next turn
Yawn vs. Electric Terrain
Yawn, then set Electric Terrain next turn (max move / Pinurchin)
Yawn vs. Misty Terrain (4th turn)
Yawn vs. Misty Terrain (5th turn)
Yawn, then set Misty Terrain next turn (max fairy)
Yawn vs. Corviknight with Electric Terrain active, then Roost
Yawn vs. Substitute
Yawn, then use Substitute
Yawn vs. Sweet Veil (all Sweet Veil is Alcremie)
Yawn, then switch in Sweet Veil
Yawn, then switch in Sweet Veil and KO Sweet Veil
Yawn vs. sleeping Pokémon
2 Pokémon remaining: Dynamax Hydreigon and Arcanine. Use Yawn into Hydreigon, then Protect. Side-WoW into Hydreigon.
Yawn into Lum Corviknight, then Sleep Powder Corviknight next turn
Record this post: https://www.smogon.com/forums/threads/ultra-sun-ultra-moon-battle-mechanics-research-read-post-2.3620030/post-8264465
Yawn vs. Hydration in rain
Faster Yawn into opposing slower mon, then slower Yawn into opposing faster mon (not the Yawn users)
Yawn Dragapult, then Phantom Force
G-Max Snooze and inflict drowsy
G-Max Snooze and don't inflict drowsy

TEST (but don't record)
Yawn vs. Insomnia, Vital Spirit, Leaf Guard
Yawn, then Skill Swap the above abilities
any of: Leech Seed / Aqua Ring / Ingrain / Curse vs. Shed Skin and Healer
any of: Leech Seed / Aqua Ring / Ingrain / Curse vs. Yawn