Protecting the Pocket
The pocket, also known as the passing area, changes depending on drop and set from the QB. In general terms, it is the area the QB needs to effectively move and step into any throw that he may make. It does differ, as I said, with various types of drops. For instance, on a 3-step drop your interior OL will try not to lose any ground. On a 7-step drop the interior OL can drop about 3 yards.
For common drops and protections, the interior OL will drop to a depth of 3 yards and are responsible for maintaining the depth of the pocket. The tackles are then responsible for the width of the pocket and will extend their depth to about 9 yards. On top of that, TEs will often assist in the protection of the edge, many times simply by slowing the speed at which the defender can gain depth and therefore squeeze the pocket. RBs, depending on their coverage, could be responsible for depth or horizontal maintenance. Typically, the RB will want to step up and into the defender, preferably setting no deeper than the heels of the OL, and never in front of the OL.
There is this idea that an offensive lineman must essentially stay within "the cylinder". This is one of those cute phrases that coaches use to try to give players a picture of what is to happen. While pass blocking, the feet and knees must always maintain a stance outside the hips and the feet should be inside the outside of the ankles. This is the cylinder and it sounds completely confusing when it's simply in writing. All it's really directing is how to place your feet relative to your body. For example, stand up and put your feet slightly wider than shoulder width. Now bend at the knees. If you have a normal stance, you'll see that your feet are still outside your hips but that your knees are generally aligned with your ankles or even outside. So to correct this, kick out your ankles so that they are parallel and once again bend at the knees. You'll notice now that your feet and knees are both outside your hips and your knees are inside the outside of your ankles. This is staying inside the cylinder.
Combined with proper weight distribution, this allows for proper center of gravity, leverage, and balance. If the knee rotates outside the ankle, for instance, the shoulder and hip open up and this creates a soft hip and soft shoulder in which a defender can drive through. Always stay inside the cylinder.
But this is just the lower body. The upper body must ideally remain inside the cylinder as well. This means the upper body should not lean, but rather the back should be flat from the waist through the shoulders vertically. Heck, even the chin should be back and the head up. Leverage comes from the hips. In every way, a pass protector blocks from the hips. While the arms may extend outward to maintain balance upon moving, when set to punch, the arms should drop down the sides of the body and tucked, the hands should be inside with the thumbs up, and the punch must extend out and up from the inside of the body. The offensive lineman will try to squeeze the rushers breast plate right around with his palms out and fingers open, as if, well, as if grabbing the rusher's breasts. So that's the cylinder. It also describes another key pass protection technique: the punch, which will be described more in a second.