What is the function of the switch which selects either SSB or CW-FM on some VHF power amplifiers?

RF amplifiers behave differently depending on the modulation type of the signal they are boosting. SSB (Single Sideband) signals vary in amplitude and have gaps during pauses in speech, requiring a slight delay in the transmit/receive relay so the amplifier doesn't rapidly click on and off between syllables. In contrast, FM and CW signals maintain a constant carrier amplitude while transmitting. Switching an amplifier to SSB ensures linear amplification and sets the proper relay delay, while the FM/CW setting optimizes the amplifier for those specific modes. This switch does not magically change the mode of the original transmitted signal; that is determined entirely by your transceiver. It merely configures the amplifier to handle the transceiver's chosen mode properly.