If your humidity is where it needs to be (which is VERY important as to whether the chicks hatch come hatch time) the air cell will grow as the moisture evaporates through the shell during the incubation period. Generally the air cell will grow more on one side. This side where it dips the lowest is generally where a correctly positioned chick will pip.
There are two stages of pips a chick makes. The first pip is unseen to hatchers (unless they candle at this point) because it is when the chick pierces the membrane under the air cell to allow them to get their beak up in the aircell. This is called an internal pip. Generally within 24 hours from this point of progression you will see the external pip, usually when hatchers mention a pip, this is what they are talking about, when the chick breaks the shell. Often it is just a bump pushed up in the cracked shell. After this pip it can take anywhere from a few hours to 24+ before they progress to zipping, which is when they actually zip around the egg and push out. During this time between pip and zip the chick is resting, strengthening lungs and the vascular system between egg and chick is shutting down. Also if a chick hasn't finished absorbing yolk before the pip, it will in most cases finish during this time. A lot happens between pip and zip and this is why assists should not be done hastily.