What does the Pokémon Go Plus use to track distance walked?

This article mentions how the Pokemon Go Plus accessory tracks your distance walked:

This is a key part of the product, as the Plus uses your phone’s capabilities to leave GPS running in the background, much like you see with navigation apps that are actively guiding you during trips.

Thus, if you are only wearing the Pokemon Go Plus accessory and do not have your phone with you, your distance walked will not be tracked.