Whistle Sprinting is a very basic technique that you will use everywhere in your speedruns. This technique allows you to run at nearly full sprinting speed without consuming stamina.
To perform this technique, hold the whistle button (d-pad down) and repeatedly tap the sprint button (B) while running.
Due to whistle sprinting being very slightly slower than regular sprinting, the most effective method of employing this technique is to alternate between regular sprinting and whistle sprinting. Run until your stamina is nearly depleted, then switch to whistle sprinting and let your stamina recharge. Once your stamina is full, change back to normal sprinting. Rinse and repeat.
Due to the awkward button requirements of this technique, most players use a claw grip in their left hand in order to effectively push the needed buttons while still having good control of the left control stick. Use your thumb to hold d-pad down and your index finger to control the left stick.