Like you, unsure All I know is that it goes for about 7-10seconds then stops. However, each time you do it (consecutively) the noise sounds like it's getting more and more "restrictive" almost like it's "filling the lines up"
Anyways, I used to do it 3 times before cranking the motor and it started each time. If not then I'd be cranking for ages and the initial idle was very bad Not enough fuel going in to the engine.