From Bulbapedia, the community-driven Pokémon encyclopedia.
Pokémon GO Plus accessory
The Pokémon GO Plus (Japanese: Pokémon GO Plus) is a peripheral for use with Pokémon GO. It became available in most countries on September 16, 2016, and was made available later in 2016 in parts of South America. It was originally announced with a release date of late July 2016.
The Pokémon GO Plus can be worn either by clipping it to clothing (or an accessory such as a belt or a bag strap) or on a wristband (both of which are included with the purchase of the Pokémon GO Plus). To switch to the wristband, users must remove the backing of the Pokémon GO Plus, which includes the clip, and replace it with the wristband, which has a backing built in. Both backings are secured with a screw, so a screwdriver is required to switch between the two. This is similar to how the Pokéwalker came with two different backings (one with a clip and one without a clip).
The Poké Ball Plus has the same functionality when used with Pokémon GO. However, if the Poké Ball Plus contains a strolling Pokémon, the device will automatically spin Photo Discs at PokéStops or Gyms without any interaction.
The Pokémon GO Plus has the same design as the marker used in Google Maps: Pokémon Challenge.
The Pokémon GO Plus communicates with the player's smartphone (or other Android/iOS device) via Bluetooth connection. It notifies the player about in-game events via multicolor LED and vibration. When the Pokémon GO Plus is in active use, the player may continue to experience gameplay elements of Pokémon GO even if they place their device in sleep mode and store it in a place like a pocket or in their handbag. The Pokémon GO app must still be open and running in the background of the player's device to use the Pokémon GO Plus.
Encountering wild Pokémon
When a wild Pokémon is nearby, the Pokémon GO Plus will start flashing and vibrate. The light will flash green if the Pokémon has been caught by the player before and will flash yellow if they haven't caught it before. Players may attempt to capture the Pokémon by pressing the button on the Pokémon GO Plus. If the capture is successful, the button on the Pokémon GO Plus will flash in multiple colours and a notification will be sent to the player's device that says "You caught a Pokémon!". If the capture is unsuccessful, the light on the Pokémon GO Plus will flash red and a notification will be sent to the player's device that says "The Pokémon fled!". The Pokémon GO Plus only allows players one attempt at capturing a Pokémon. If the capture is unsuccessful, the Pokémon will flee immediately.
When attempting to capture a Pokémon, one Poké Ball is depleted from the player's inventory. The Pokémon GO Plus will only use Poké Balls — not Great Balls or Ultra Balls. If a player encounters a wild Pokémon and attempts to capture it when they have no Poké Balls, the light on the Pokémon GO Plus will flash red just as if they had tried and failed to capture the Pokémon.
All Pokémon encountered while using the Pokémon GO Plus will appear in the player's journal in Pokémon GO. Players may review their journal to determine which Pokémon they captured or missed out on while they were using the Pokémon GO Plus.
The Pokémon GO Plus will flash blue and vibrate when within range of a PokéStop. Players may collect items and XP from the PokéStop by pressing the button on the Pokémon GO Plus. Items acquired will be added to the player's Bag and a notification will be sent to the player's device stating how many items they collected (e.g. "You received 3 items from the PokéStop."). If a player's Bag is full, they will still gain XP as they normally would while playing Pokémon GO.
Similarly to Pokémon encounters, players may review their journal to determine which items they obtained from PokéStops while they were playing with the Pokémon GO Plus.
Pokémon GO Plus with wrist strap
North American Pokémon GO Plus cover
European Pokémon GO Plus cover