What does it mean that routine replaces willpower?

Established routines execute without willpower expenditure.

Example: A writer spends three months agonizing over when to write, where to write, whether she feels inspired, and how long each session should last. Every morning involves a negotiation between the part of her that wants to produce pages and the part that wants to check email, tidy the apartment, or wait for a better mood. She averages two sessions per week, each preceded by forty minutes of deliberation and guilt. Then she installs a routine: every weekday at 6 AM, she sits at the same desk, opens the same document, and writes for ninety minutes before doing anything else. The first two weeks are brutal — the routine demands the very willpower it will eventually replace. By week six, she no longer deliberates. The alarm fires, her body moves to the desk, and the writing begins before her conscious mind has formed an opinion about whether today feels like a writing day. She now writes five days a week, has tripled her output, and reports that the willpower she used to burn on the daily "should I write" negotiation is available for the actual craft of writing — word choices, structural decisions, and the difficult passages that genuinely require deliberation.

Try this: Identify one behavior you currently perform inconsistently because it requires a daily willpower decision — exercise, creative work, journaling, studying, meal preparation, or any recurring action that you want to do but frequently negotiate yourself out of. Design a routine with four fixed parameters: same time, same location, same trigger (the action immediately preceding it), and same minimal duration. Write these four parameters on a card and commit to executing the routine for twenty-one consecutive days without deviation and without evaluating whether it is "working." At the end of twenty-one days, rate the willpower cost of the behavior on a scale of one to ten, where ten is maximum deliberation and one is fully automatic. Compare this to your estimate of the willpower cost before the routine was installed. The difference is your willpower dividend — the recurring savings produced by the formation investment.