Hi
When used upright, the idea is that the 'dirty' water is fed through the top nozzle down the tube to the bottom of the vessel; it then exits the bottom of the tube and percolates upwards through the resin getting cleaned/polished as it rises - then exiting the top nozzle as pure water.
If laid on its side, the water will not travel through as much resin and the water exiting will not be as pure as that when used upright.
Give it a try and see.
Cheers, Tom