Was working on a vintage Bryant outdoor unit yesterday - installed around 1975. It was a rectangular unit with the fan on one side, and a flat coil on the other. I just assumed that the fan was "pulling" the air through the coil, so that the inside of the housing would be under a negative pressure, but instead, it was the opposite - fan sucking in, and blowing through the housing and out the coil resulting in quite a positive pressure.
Well, when I slid the cover off, (unit running) it blew off and hit me right on the bridge of my nose so hard that I saw stars. The only thing that kept me from passing out was the pile of dried up leaves inside that blew out and went into my eyes.
Ya learn something every day.