It wasn't until after the session that I understood what had happened and I only thought it through because I was wondering what was making me grin so long my face hurt.
The transition from new person to RPGs to Player is a a subtle change. It has nothing to do with knowing what die to roll, the rules, or where to look for stuff in the Player's Handbook. It happens the first time a player figures out how to transcend the rules and make their characters more effective than they have any right to be on paper. That first wacky plan or strategy that bends what you have on the character sheet into something incredible. That's the moment when they stop imagining the board game structure in their heads and realize they can try nearly anything.
My gang is running through the Caves of Chaos in old Gazetteer-era Mystara. The party is small, with only three of them so they explore with great stealth and care. A common tactic is to send the party wizard's spider familiar ahead to recce the hallways and rooms before entering. The spider can get under most (but certainly not all) doors in the rough hewn caves and that makes the difference between a stealthy retreat and a TPK.
They knew they couldn't take it in a straight up fight. The party has had close calls with goblins and it's obvious the hobgoblins are far tougher than the goblins. They started looking at their sheets and asking questions. The teifling could cast Thaumaturgy which, among other harmless effects, makes tremors in the floor. "That's pretty scary in a cave, right?"
The wizard had cast illusions in the past to improve their chances to hide from patrols in the valley so they asked some questions about how the tremors mixed with illusions of rocks falling out of the ceiling might work and the bold gambit was hatched: They would fake a cave-in and strike in the chaos. An elven thief, a tiefling monk and a gnomish abjurer against 13 hobgoblins.
I live in a mining town. Work occasionally takes me into the mines and I've been down as far 7,400 feet underground. While I have never experienced a cave-in (and I hope I never do), you can't go down to any depth without being aware of how dangerous it is. While sounds of tremors coming from the walls would not create panic in hardened warriors, it would induce immediate action. Adding falling rocks would create some panic and a lot of action.
With the hobgoblins spread throughout the room I decided their actions would be based on where they were. The two closest to the door the party was coming from would exit and run down the hall, away from the cave-in. The two closest to the barred door would get the door open and get out and down the stairs. The rest would try to get under the table or press themselves against the walls and between crates for safety.
Despite the monk never hitting once with his spear the entire session (he even dropped it once), the surprise round went well. Two hobgoblins went down to a sleep spell (and looked like they were crushed by rock!), another one dropped to the thief's sneak attack with an arrow and a fourth reeled from a nasty kick by the monk. I ruled that the hobgoblins couldn't use their martial prowess ability that gets them extra damage on each hit when fighting in groups because they were too distracted by the falling rocks to work together effectively. The removal of that advantage was key to the party survival as they took some lumps but didn't lose anyone. With the hobgoblins engaged in escaping the room the party was also able to attack them in smaller, more manageable groups.
The party won, quickly grabbed some loot and then ran for it through the goblin caves (and four goblins) to get away from the hobgoblin reinforcements coming to help after the cave in.
Certainly some critical rolls went in their favour, but the conception and execution of a plan that basically tripled the effectiveness of their plucky little band marks their graduation to seasoned RPG players. Now they'll be searching their environment for advantages and trying all kinds of crazy stuff to turn the tables on their more powerful foes. That is where player skill changes the game.
I know they've made the switch because once they got back to the Keep they started reviewing everything they know about the different factions they've encountered and how they relate to each other. They are discussing how they can make use of that.
From a DM's point of view the campaign levelled up. I need to be ready for all kinds of crazy stuff from here on out.
I couldn't be happier.