Derpy will follow the bubbles that you control, but only if they are in her direct line of sight (Derpy will literally brighten up when she sees them).
You can make Derpy run by pressing and holding the left mouse button to increases the bubble's power, but do it for too long and the main bubble will pop. It recharges after a while, but in the meantime it casts no light so Derpy won't see it.
Amount of smaller bubbles = amount of lives.
Derpy will get bored if you stay in one place for too long; same if she doesn't see the bubbles.
Derpy will not walk into any harm on her own.
WARNING! The game requires powerful CPU so if you can't play it, here is a complete walkthrough: [link]
UPDATE 22.09.2012: The site-lock has been removed so the game can be played offline now. If SWF file won't work alternatively an EXE file is available here: [link]
People that made this game at least 20% cooler:
Forest Rain by composing awesome music and SFX for cutscenes.
Decibelle by lending her wonderful voice to Derpy
Resonant Waves [link] by allowing me to use the lovely "Luna's Palace" and "The Temple of Rain" tracks.
PonyVisation [link] by allowing me to use the great "Everfree Shadows" track.
And of course, special thanks to RizCifra for creating the inspiring "We Are Pilots".