Hand driven sirens are more often used in situations where electricity has been affected and so an electronic siren is out of use – for instance during earthquakes, tsunamis, floods etc. Although in some cases, they are even just used to indicate the time of day.
A hand crank siren is just that, it is a handheld device that requires no electricity and has a distinctive handle or crank at the back of the main head. They work by manually turning or cranking the handle a few times to produce a sound slowly getting louder with every turn. Inside has a motor that drives the shaft with an attached rotor to both ends. In every rotor surround a stator, a sort of casing, which has a number of holes that is almost rectangle in shape. Each of the blades draws and forces air out through the slots inside the stator while the plates at the end of each of the blades stop or interrupt the flow. As a result, it creates rarefied and alternating compressed air pressure that produces a sound.
Hand sirens became very popular to use during the Second World War to warn residents of an oncoming air raid attack and in more recent years have been used to warn of an incoming natural calamity. Due to them not requiring power, they are very versatile and so can be used for many different applications and situations with their simple and maintenance free design.
See more here: https://beaconlamps.com/products/audible-warning-signals/hand-cranked-sirens/