Movement
Movement is a vital COD can be difficult. Have to push forward while staying alive, making sure no one is behind you while maintaining a steady pace takes practice and experience. As you play the game more, you'll learn the best routes to take and how to navigate a map so the enemies are where you expect them to be. Maps in COD are very different in their make, the creators of the franchise wanted to try to emulate different battle grounds that might come across in war, because of this the maps in COD are very complex in their design.
The lesson about movement begins around 5:06 minutes in.
A moving target is a harder target to hit, if your in one spot the entire time your playing the game, your opponents will know exactly where you are at all times. Once your position is known you become the target, either your enemies will avoid you or they will swarm you, giving you no chance to defend yourself. Constant movement is a key factor to being a good COD player.
Drop-shooting
Drop-shooting would fall under movement, the act of dropping your avatars body to the prone position in a fire fight. This is a very powerful tool in COD, because it affects the auto aim feature in COD. When someone aims to fire at a enemy in COD, the game will automatically aim at the nearest target to the direction your pointing at, though it is very subtle feeling when playing. Once the player is locked onto an opponent, the auto aim feature is not in effect anymore, so it is up to the player to make corrections to the position of the cursor afterwards, what drop-shooting does is it drastically changes the position of your avatars body while making it a smaller target to hit. This tactic works better in close range combat, but it is not useless at a further distance.
As mentioned in the video, if you dropshot too much, skilled players will begin to aim lower to the floor. The point of drop-shooting is take advantage of the auto aim feature found in COD, a feature that automatically, when aimed upon someone, will set the cursor to the enemies chest. Drop-shooting moves your avatars chest to the floor, making it so your opponent has to manually aim DOWN. If you dropshot too often, your opponent will start aiming down everytime they get into a firefight with you. A disadvantage to drop-shooting is the accuracy of your bullets severely goes down, so if your opponent is making corrections to his game and starts to aim lower, your accuracy won't be able to beat his. Use in moderation.