Here I explain the differences of running an aquarium pump when used internally (submerged) and when used externally.
The basic concept of having water within the impeller chamber is what helps the pump start. This is because the impeller is designed to pump water, hence it will not work when running ‘dry’.
This method of priming the pump is only one of a few other possible solutions.
The theory of priming the pump also applies to External Canister filters, although there are some differences.