Dive
Dives are performed by pressing DOWN while in the Running Jump state. This sets Peppino's vertical speed to -10.0 while keeping his horizontal speed constant. Dives are still subject to Gravity, so Peppino's vertical speed may reach -20.0 if he falls for long enough. Peppino's collision also becomes smaller during a dive, enabling Gap Dives.
Once you begin a dive you are mostly committed to it, since you can't interrupt the dive or change the directions until you touch the ground or bonk into a wall. Your only movement option at this point is to link the dive into a Powerbomb by pressing JUMP.
Dives are not only useful for maneuvering though levels. Since during a Mach Dash Peppino can only accelerate while on the ground, diving can be helpful to stick to the ground for longer to build up more speed.