The moment you tap the viewfinder to place an object, Motion Stills plunks an object in that scene with the assumption that the surface in question is parallel to the ground, using the phone's accelerometer as a reference point. It then separates the camera's rotation and translation estimates to create a 3D translation, and combines that with gyroscopic data to give you a truly perspective-correct view of virtual objects, even if you pan away and come back. You don't need to calibrate it, and it should work on any Android device with a gyroscope (that is, you shouldn't need ARCore).

There's no mention of if or when this might be available for iOS users. That might require considerably more work if it happens, since the current implementation is dependent on Android tech. If you do have an Android phone, however, this gives you an easy way to spice up clips that you can share just about anywhere, not just on your social network of choice.