The one idea under everything you make.

Most recipes hand you steps with no model, so when it fails it's a mystery and you go hunting for a better recipe. The thorough ones swing the other way and bury you in every variable, until you're trusting their testing instead of your own understanding.

These do neither. You enter by your job, learn the one mechanism that governs it, and leave able to reason about the rest yourself. Deep on the one idea. Ruthless about everything else.