after researching and asking people i think i understood it now, so correct me if im wrong.
underspeed means if we're shutting off the engine the frequency of the generator frequency goes down and that would trip all the bus tie breakers in a parrallel system due to under frequency protection activation so a speed switch is situated in the CSD to monitor the oil pressure if it goes low due to underspeed ( engine shut off ) it transmits a signal to GCU which trips the generator line contactor of that particular generator only without tripping all the bus tue contactors and all the other generators .
please someone correct me if im wrong.