There's nothing worse than getting hit by a car riding a bike (but cars can't ride bikes!). Kidding, there are much worse things, like getting hit by a tractor trailer while riding a bike. Missiles and gunfire are also bad. The helmet design is dubbed Hövding and while consistently described as "invisible" is just really cleverly hidden:
Hövding is a discreet collar that the cyclist is wearing around his neck. The collar contains a folded airbag which is visible only at a collision. The airbag is designed as a hood that in case of an accident will enclose and protect the cyclist's head. Release mechanism is controlled by sensors that register abnormal movements of the rider in an accident.
A device like this is certainly appealing and would keep helmet-hair away, but I do wonder whether it could cause injuries in the same manner as an airbag in a car.