Can lines be self draining?

[hk_ont]

Is it always necessary to blow out the pipes with air? The reason I ask is that in the past two closings, done by my pool company, I never saw them blow out the lines. They just drained the water below the level of the returns and I assume the water just ran out via gravity. Does this make sense?

If I do blow out the lines, how do you control which pipe is being blown out? If the compressed air spreads out into the various pipes, I wouldn't think there'd be enough pressure to blow out all the water. I was thinking of installing all the rubber plugs and then removing one at a time to concentrate the air into that pipe.

Any comments?