FRSWITCH is a switch that turns on frequency switching. If specified, a second group of synthesizer settings should be given in BBSYN2. This is used for VLBA files only. In fact, the program will die if frequency switching is attempted when a VEX output file is required.
In general, frequency switching is probably better handled with a scan loop. The FRSWSITCH option is not used often, if ever, and should only be used with care.