pool return outlet sucking water

[btaylor]

My pool plumbing has 3 return valves/pipes: one goes to the waterfall, one to the lounge where it creates a bubbler-effect, and the third goes to the three pool wall outlets. Occasionally I turn off the valve to the wall outlets. This causes the waterfall and bubbler to be stronger, which looks and sounds nice. But then the wall outlets suck water back toward the filter. Is there a logical reason for this? Could it be causing damage to my pump/filter? Is there any downside to running the system with that wall outlet valve turned off?

[Guest]

If you turn off the valve the line should be isolated and should not suck or blow any water at all.

Could you explain this "sucking".