How does your video game controller know when to vibrate

When something happens in-game that is suppose to make your controller vibrate, or "rumble", the code in the game sends a message to your controller to tell it to send a current of electricity through the "rumble-pak" - making it feel like it vibrates.
It wasnt until early 2000 that the first standard controller came with a build-in Rumble Pak. Before that not every game controller could rumble, some controllers that allowed for an add-on devices to be connected to the controller, such as the Dreamcast and Nintendo 64, had small things that was called "Rumble Paks" that you stuck in to a slot on the controller to make it vibrate when the games told it to.
Not every game supported a Rumble Pak though so it was often described as a feature on the box saying that a rumble pak could be used with the game.