| It is simply a pot that includes a DPDT on-on switch. Pretty handy little thing actually.
DPDT means Double Pole, Double Throw. Double pole means there are two sets of independant poles so two circuits can be switched. Double throw means both are activated in the same motion. On-on means there is a circuit closed in either direction. It is a two-position switch.
These are very common switches with a variety of applications. This one is nice because you wouldn't have to drill another hole in your bass to install it.
Just a few things you might use a DPDT for include:
1. Series/parallel switch on a jazz bass
2. Active/passive switch to bypass onboard preamp
3. Bass or treble boost as part of a particular preamp design
There are countless other things you could do with it depending on what you ultimately want to engineer | 
