The trainers are designed to sort out VR's "limited space" problem with motorised VR shoes.
Google's patent 2018/0326286 A1 describes what are essentially motorised VR roller skates that will let the user walk normally while the motors and wheels work to negate your natural locomotion and keep you inside the VR safe zone.
The trainers will let you walk "seemingly endlessly in the virtual environment" while keeping you in one spot in real life. Google's shoe solution would track the user's feet, just like how VR controllers are tracked today. The tracking would know when you're too close to the virtual walls of your VR area, and the system would wheel you back into place.
Patents are always written to give the broadest possible coverage of an idea, but Google's patent shows normal wheels, tracks, and even omnidirectional mecanum wheels as possible wheels for the VR shoe bottoms. Omnidirectional wheels would be great, as they would allow you to do things like sidestep, while still having your position corrected by the shoes.
It will be interesting to see how many people will killed by falling over or tripping over the cat. VR treadmills out there have a rigid waist support, in part to keep users upright if they stumble. Adding a set of wheels to the bottom of your shoes, which could start and stop unpredictably, may make staying upright a challenge.