Welcome to the age of Soli. Google has shown us how the Soli Sensor will do a couple of neat tricks in the Pixel 4, and while waving your hand like a Jedi to change what song is playing is pretty self-explanatory (and honestly, not that exciting). Using it in tandem with dedicated hardware to unlock your phone with your face is something we haven't seen before.
This isn't the face unlocking you're used to seeing.
Facial unlocking, however, is something we've seen before. First debuting on Android 4.0 Ice Cream Sandwich with the Galaxy Nexus, using your phone's selfie camera — or, in newer iterations, infrared cameras— to unlock your phone is an incredible experience. It's also reasonably fast, easy to use, and until Apple developed Face ID on the iPhone X, extremely insecure. Face unlock that only uses a camera to capture data on a two-dimensional plane is easy to fool with a photo. That made it more of a convenience feature, one even Google said shouldn't be used if you value security.
I've previously written about the tech that drives Face ID; those same words apply to Google's implementation on Pixel 4. That's not surprising as it's a well-established technology that can map and recreate a 3D image of basically anything. The only amount of detail captured is only limited by how long you want to wait for the results; the more data obtained, and the more time allowed to process it, the better and more accurate the results can be. But most people don't want to wait that long to unlock their phones, which is why companies need to compromise between speed and security.
A quick refresher on how Face ID works: using a special LED that "washes" your face in infrared and near-infrared light, along with a unique component that projects a matrix of contrasting IR "dots" on your face, cameras collect everything in front of it and build a 3D map. This map converted into a unique token based on the shapes present (usually a face, or part of it), and that token used to check against the data stored inside a secure element on the phone. If the two signatures match, the phone unlocks.