It’s actually a driver on the back…
“WHY DRIVERS ON THE BACK OF THE SPEAKER?
During our research for the A500 and A700, we have tried a lot of different configurations, to obtain the performance we wanted. In the end, we ended up placing some drivers on the backside of the speakers. This has two purposes, from which only one of them is obvious. The obvious reason is that we want to have as much cone area as possible to be able to extent the bass-reproduction as much as possible with as little distortion as possible. The second reason is, that we use the rear drivers with a specially developed algorithm to perfectly align them with the remaining front-firing drivers, without causing any spatial problems to the acoustic dispersion. This allows us to use rear wall reflection to extend our sound image even further than traditional loudspeakers. The result from this is a larger and more precise sound image than you can achieve with traditional front-firing speakers.”
More details here: