His thinking is to uncap the bleed tube(which is at the high point and even with his fill tube) and let the air come out from there. Then he puts the water in through his fill tube, which feeds into the lowest point.
Think of it this way, ever tried to put liquid into the single small opening of a bottle? It's a biatch, because the air in the bottle has to escape from the same opening the liquid is going into.
Maybe an even better example is a gas can. If you don't open the vent cap, the gas comes out slowly because the air has to go back in to the same opening the gas is coming out of. Open the vent cap and the gas pours out, while the air rushes in through the vent. His idea is the same thing, only in reverse (liquid going in, air coming out), with the liquid "pushing" into the system at the lowest point and the air going out at the highest.
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