Bhagavad Gita, Chapter 6, Verse 24

Abandoning without reserve all desires born of Sankalpa, and completely restraining, by the mind alone, the whole group of senses from their objects in all directions.

Letting go of a desire doesn't usually happen all at once, it happens through steady, repeated redirection.

This describes real, ongoing effort, not a single decision. Actively drawing the mind back from where it keeps wandering is the actual mechanism being described, not something that happens automatically.