Intermediate switch wiring diagram (old cable colours)
Intermediate switch wiring diagram (new cable colours)
The intermediate light switch is used where three or more switches control one light and used in conjunction with two two way light switches to achieve this. It is effectively two single gang intermediate switches on one face plate.
Light switch mechanism
Each of the gangs (or switches) above in Fig 2 (of which there are two) work like this (Fig 3): There are four terminals, usually L1, L1 and L2, L2. In one position, L1 and L2 are connected in pairs. In the other position, alternate pairs are connected.