The reloading system in Call of Duty: Advanced Warfare is rely unique, taking the perk ‘Slight of Hand’ (From previous Call of Duty games) and putting it into the reload system without the need of the perk.
There is the standard reload where you press the reload button and it reloads your weapon, putting what ever bullets you had left in your weapon and adding it back to you bullet count.
But there is also a second type of reload.
If you double tap the reload button, you will do a slightly faster reload, but the bullets that yu had in there clip at the time of the reload, will be discarded and not added back into you bullet count like on the first type of reload.
Try it for yourself if you do not understand, but i think hat it is a greta little thing that they have added into this game, giving you the option to do a quicker reload if you need it if you are in a sticky situation, or a slower reload.