That is correct mate. I'm sure someone can explain this better than me but basically if it's under water the pressure zones stop working. I've been in some tight spots with the McKirk whereby the last riffle wasa little submerged, can't get everything perfect the first time.
Old Jeff Williams has a you tube video that really lacks the detail he puts in but from the suggestions tab you should find a few suitable videos. There's one with a bloke bashing the sluice in his garage woeful production values (hard to watch and narration would nearly put you to sleep) but the old bloke knows his sluicing set ups. I'll see what I dig up.
Found it mate the bloke goes under prospectingfool. Let me know what you think about his red stick presentation.