From the manual:
Track Mode is always disabled when you start Model 3. To enable Track Mode for your current drive, shift into Park, then touch Controls > Driving > Track Mode. When enabled, TRACK displays on the touchscreen above the driving speed. You can disable Track Mode at any time by turning the setting off on the touchscreen.
Track Mode remains on even after you exit your vehicle to quickly cool powertrain systems between driving sessions. Track Mode automatically turns off when the vehicle is sufficiently cooled or if the vehicle is powered off and you power it back on again.