It's a two stroke boxer motor with a second set of outboard pistons running opposite to the primary units.
The only real big changes worth noting over existing auto engines - it's a two stroke, and they have it running off a turbo decently.
If you were to make a modern DFI two stroke with their turbo you would see the same compactness and emissions and even FEWER moving parts. The double opposed setup is just flash to distract you from the fact that it's a two stroke.
Now, combine said theoretical DFI motor with Polimotor's research, aka a nearly all plastic motor and you'll see the power density go through the roof. THATs the engine I want.