From B&W's website:
There is a large aluminium slotted screw at the back of my speaker, behind the FST midrange. What is it for and should I adjust it?
In many products, our FST midrange unit has an unusual mounting arrangement, where the driver is mechanically isolated from its enclosure by flexible gel components in the fixing arrangement. To optimise the isolation, a single securing rod is used that runs from the back of the driver to the back of the enclosure. The tension on the rod is controlled by a large headed, straight slot screw at the back of the enclosure, and this in turn controls the pressure on the flexible gel parts.
The amount of tension varies between different models and in some cases it is necessary to prevent excessive movement of the driver should it be exposed to impact in transit. In such cases, notably 800 Series products having the sphere/tube midrange enclosure, a clamping disc is fitted over the screw head, held in place by a crosshead screw in the centre. On installation, this clamping disc and centre screw should be removed in accordance with the pictorial instructions on the label fitted to the assembly.
It is important that the tension on the rod is set correctly. It is preset at the factory and should not require adjustment, except if the driver has to be removed for one reason or another, but sometimes it may get altered by mistake. This may be because one's natural instinct is to have a fiddle to see what happens, but occasionally removing the transit clamping disc has been known to loosen the main screw if it has bound itself in the thread.
If either of these has happened, please refer to your local B&W dealer to have the tension reset correctly. We recommend that you do not attempt this yourself.