If you have the Mega Man Mega Buster Gun Replica, you already know how this works, but for those who don't, here's a short tutorial.
Turn it on and you'll hear a power up sound and watch the power meter fill up. Reach in, grab the handle, and you're ready for action. Quickly pull the trigger and you'll hear a single fire sound and the end will flash. Hold in the trigger, though, and you'll watch the power meter fill and the sound intensify until you release the trigger, firing a charged shot! When you are finished vanquishing evil robots for the day, switch the Proto Buster off and enjoy the power down lights and sounds. There's a new sheriff in Monsteropolis - and it's you!