The non-self-priming kind like March pumps use an impeller. That kind of pump can't pump air, so it has to be primed (the pump head must be filled with liquid) to push anything. Impeller pumps can push decently but they have practically no suction so you have to make sure you have a constant flow of liquid to the pump head by having the source above the pump so gravity keeps it full.
If you get air in the pump head it'll stop pushing, so you can cut the power and let the bubbles rise up out of the head and reconnect the power. Usually it takes me two or three pulses to get it going.
One nice thing about March pumps, though, is that they're magnetically coupled. This means you can restrict the outflow to slow it without damaging the pump. Restricting the outflow is also good for helping to keep the pump primed. You never want to restrict the flow on the inlet side.