One important thing to note about power heads is that they almost never have sump capabilities - in other words, they are incapable of sucking water up a tube if there is a mass of air between the water and the impeller. What this means is that these pumps must be immersed at least up to the impeller housing.

The reason why a power head can only move water is the impeller design. Basically, you have a small electrical motor sealed in epoxy (so that the electrical bits don't touch the water) with a small plastic fan-like bit on the end. The "blades" may be angled, or straight up and down. Water enters through the bottom of the pump, where the impeller's rotating blades hurl the water against the side of the impeller housing using centrifugal force. There is a hole at the side of the housing where the now-pressurized water exits. Water has much higher viscosity than air, and is relatively easy to pump in this way - but air, being of relatively low viscosity, is very hard to pump in this manner. (Air is much easier to pump using a radial or axial turbine.)

If you need to use a power head as an inline pump (like I do), the best solution is to either mount it at the top of your project's reservoir or, in the case of a power head with a circular impeller housing, find a pipe that's as wide as the impeller housing and figure out a way to integrate it into the project.