When a car approaches from behind you get a visualisation on the bike computer showing you how far away the car is and an audio alert. If the car is approaching fast you get a different audio alert and the light flashes differently to warn the approaching car. It also detects up to three cars.
There might be improvements to be had but the implementation is pretty solid.
There's one feature they all miss. They only measure relative velocity to You. That's definitely better than nothing, but in my experience the most dangerous drivers aren't necessarily the fastest ones, but those who pass too closely. So, something like "angle of approach" measurement would be a feature I'd personally be willing to switch my Varia for something else.