You can change the level depending on what time the clock is showing when you step into the course. As the minute hand moves around the clock the level gets faster and faster—if you enter the level ...
Mario 64 Walkthrough (All Stars) - Tick Tock Clock: The Pit And The Pendulum IGN's guide to completing Tick Tock Clock - The Pit And The Pendulum in Super Mario 64 and Super Mario 3D All-Stars.