i have an older pool (fox installed in the early 80s..inground vinyl) with main drain and skimmer. i've never worried about main drain being active while someone is in the pool until i read posts here. putting my feet on the main drain, there never appears to be any suction to speak of..but i do know it's working fine confirmed by turning off my skimmer valvel.water flows fine with no bubbles through the main.

i'm frankly more worried about having my pool light on while swimming after seeing that stupid george clooney movie, sarinta or whatever it's called

(if you haven't..don't..it'll freak you out about pool lights)