A Google spokesperson said that the phone can reach 10W charging speeds on a Pixel Stand thanks to a "secure handshake," an extra layer of security that also allows the Pixel 3 to show the Assistant UI on its screen while docked. Turns out there's more to it than that, though: Google also said that it's certifying chargers for the new flagship via the "Made for Google" program.
The tech giant pointed Ars Technica to a Belkin wireless charger that claims it can reach 10W charging speeds with the Pixel 3, because it was "specifically engineered to optimize performance for" the device. Its product page clearly says that it "uses Google's 10W proprietary wireless charging technology." Unfortunately, it's not yet clear if that technology can be retroactively applied to existing charging pads or if Google can only certify new chargers for its phone.
Update: A Google spokesperson gave the following statement to Engadget.
"We don't limit third-party devices, in fact, we're working with our partners in the Made for Google program to get fast 10W chargers certified for use with Pixel 3 (Belkin announced its 10W Pixel 3 charger already which will launch in the coming weeks). Pixel Stand and Pixel 3 work together through the protocol we've developed for fast charging. Everything else charges at the industry standard Qi 5W."