The little button will vibrate and light up green when you encounter a Pokémon. You can click the button to throw a Pokéball. If you catch it, it will light up and vibrate! If it flashes red, the Pokémon escaped.
It will also blink and vibrate blue when you are near a Pokéstop. If you click the device's main button, the Pokéstop's items will be added to your inventory.
There are some conflicting reports about what kinds of Pokémon the Plus device will let you catch.
Previously, Niantic said the accessory would only let you catch Pokémon that you had already encountered and added to your Pokédex in the full version of the app.
On Reddit, however, some people who have the device already are saying that they have been able to catch Pokémon that they hadn't yet encountered. We'll update this post when we know more.
It seems to only throw standard Pokéballs, which will probably result in a lot of fleeing Pokémon. Also, you can only throw one single Pokéball at each Pokémon. If you don't catch it on your first try, you're out of luck.
[Pokemon Go Plus] the Plus uses only regular Poké Balls, and basically everything flees... from pokemongo
Reddit user Brichs uploaded the above image, showing that several Pokémon in a row fled from capture — probably because the Pokémon Go Plus can only throw normal Pokéballs.