Anyone feel free to correct me, this is my understanding on it.
My bike has one, which was fitted by the previous owner. You buy a cleaning kit, this has a cleaner in which you wash your filter with and then reprime the filter with the other tube in the kit.
However, the main selling point of these is that you get an improved airflow and this gets better the dirtier the filter (?), but the recommended cleaning interval is something like 50k miles anyway.
Steve