What changes for your Max Battles
Visiting a Power Spot or placing a Pokémon at a Power Spot after winning a Max Battle no longer triggers special wild encounters around that Power Spot. The goal is the same wild encounters for every player.
The Explorer Gadget during your session
It caught Pokémon and spun PokéStops while Pokémon GO stayed closed. Now it also helps while you play. From level 20, turn it on from its icon at the top right of the map view, or from the Key Items section of your Item Bag.
A pink Poké Ball icon then appears over the Pokémon and PokéStops it is handling. Tap one of them to take over yourself: you only have a short window to do so. Its icon also shows a summary of what it caught and how many PokéStops it spun.
It will not try to catch mighty Pokémon, or Pokémon still missing from your Pokédex. With an accessory such as Pokémon GO Plus +, you do not need to unpair it, but the two cannot be active at the same time.
The other tweaks: a clearer catch summary, a dedicated Appraise button on a Pokémon’s summary page, a new ring around Pokémon drawn by a Lure Module or met on a Route, new colors for spun PokéStops, and new effects for the Mystery Box and the Coin Bag. Everything is in the official announcement.