Open up the playbook

When a person who is currently sandbagging in an activity suddenly decides to give their full skill/effort into it.

The term originates from American football coaches tending to have conservative play calling until a game-changing scenario shows up, by which they "open up their playbooks" (e.g., trick plays) to gain an edge over their opponent.

The pro Mario Kart player had to open up the playbook against his amateur friends after finishing no higher than 3rd after every race.