What is the function of the SSB/CW-FM switch on a VHF power amplifier?

An RF power amplifier often has a switch for different emission modes like SSB/CW and FM because these modes require different amplifier characteristics. FM is a continuous carrier mode that can use a highly efficient Class C amplifier. However, Single Sideband (SSB) requires a linear amplifier so the amplitude variations of the voice signal are not distorted. Switching between these modes changes the amplifier's internal biasing and transmit-receive relay delay settings. For example, it keeps relays engaged during the natural pauses in CW or SSB speech. The switch does not actually generate or change the transmitted mode itself; your transceiver does that.