This glitch is most effective with scoped weapons like the sniper rifle and pistol, and is especially useful in multiplayer. After expending a full magazine from a sniper rifle or pistol, the animation is quite long, and in a Team Snipers gametype, the time spent reloading can make a huge difference. Rather than waiting, as soon as the new magazine is in the weapon, press your melee button/key. Instantly after doing so, you can press your scope button/key and immediately start firing rather than waiting for the reload animation to finish.