| posts: 4,814 | Location: Australia
Put your hand in front of your face, a few cm's away from your mouth. Breath in. That is the equivalent of the 'pull' fan. Now blow out. That is the equivalent of the 'push' fan. Basically the pull fan will pull air from the most direct route and of least resistance, which is from the sides (like with your hand). What this means is the front fins may not get good airflow over them. When the fans are in a push/pull configuration, the pull fan still sucks air from the heatsink fan, but since the push fan is operating, the pull fan also draws air from the front of the heatsink, which reduces the resistance of the push fan and increases air flow (and air flow/fin coverage). Since the push fan still pulls from the front side (pulls from the side) you don't really feel the airflow, and since the pull fan still pushes on the exit side, you feel the air there (like blowing on your hand).
Hope that makes sense!