Wot 'e said. unless you're in a seafront pub and a lifeboat is passing with its DF equipment set to 121.5, most likely nothing will happen if you set this watch off. COSPAS/SARSAT no longer monitors this frequency. However it is strange that the documentation referes to aeronautical situations specifically. This can only be some internal policy of Breitling's, to do with their flying assoications. 121.5 is also used for homing signals on EPIRBs and man-overboard systems so the frequency itself is used in several types of device.