Bass reconstruction processor question

hi

i found some devices in internet with bass reconstruction processor ...

audio control epicenter or soundstream BX-10 ... the PCB is sometimes similar to these devices ..
so i think there are only 1 one or two originals and the sell it like "oem" devices
because the PCB looks nearly similar

but my question is ...

how does it work ?
i did not found any schematic to see whats happend with the signal
In some pictures there is no digital part so it is fully analoge