Find the fun
Also said as Following the fun
Prototyping until you discover what is actually enjoyable in an idea, and then building towards that rather than towards the original plan.
The fun is often not where the plan said it would be. Teams who protect the plan can spend years polishing the part nobody enjoys.
Say it
“Nobody in the playtest cared about the racing, but everyone laughed when the cops crashed into them. Stop tuning lap times and make a prototype that is only police chases.”
Where you’ve seen it
Grand Theft Auto began at DMA Design as Race 'n' Chase. As its developers tell it, a bug made police cars ram the player, testers enjoyed that far more than the racing, and the team built the game around it.
Not to be confused with polish
Polish makes a thing that is already good feel better. Finding the fun is working out which thing that is, and it has to come first.