During WWI airplanes were new technology. Everyone wanted to use airplanes in war. People were trying to figure out effective use of airplanes in combat.People tried to attached large machinegun on airplanes but the problem was they couldn't fire through the propeller. So that aiming direction of plane and gun has to be different.This caused its effectiveness.And the solution to this problem was somehow linking gun and propeller through any mechanism.
You all might have seen war movies from WWII where fighter airplanes fire bullet and each bullet crossed through without hitting rotating propeller blades. This was all possible only through a gear called synchronizing gear. It not only helped to mount fix gun on an airplane but also changed the course of War. Synchronization was to be done between the rate of bullets fired from a gun and the speed of whirling propeller. The trigger of the gun was linked to propeller through gear and the firing was done just when the blades passed. The solution was timing.
above figure illustrates that the bullets passed through the safe zone without hitting the propeller.