A one way switch has two terminals, one is for the live and the other one is for the return wire back to the light. A two way switch has three terminals. One is marked common which is for the live and the other two are marked L1 & L2. A two way switch can be used as a one way by using Com & L1 or Com and L2. When used as a two way the live goes into the Com. and L1 & L2 are connected to another two way switch thereby transfering the power across both switches. When a one way switch is switched off the other terminal is dead. When a two way switch is used as a replacement for a one way, L2 is live when the swich is off but because there is no wire connected to it it appears dormant.