How Pressure Assist Toilets Work
The only thing you'll notice that's different about a pressure assist toilet is what you see when you open the toilet tank. There is no water, just a sealed plastic inner tank vessel.
The pressure assist toilet works by compressing air in the storage tank as water (under pressure in your pipes) fills the storage tank. Then once the storage tank is charged, it blows the water out under pressure like you blow water from a straw.
This creates a strong flow of water that cleans the bowl better, removes waste better and flushes further than a gravity fed system. In fact, pressure assist toilets like the Kohler Pressure Lite can flush 50% further than gravity toilets resulting in cleaner waste pipes and less chance of clogs down the road.
This is important because as you reduce water flushing volume in a toilet not intended to have the reduced volume, whether by placing a filled 2 liter bottle or brick in the water tank or just using a partial flush, the waste material may not be fully carried to your vertical soil stack.
(See the "Drain Carry Performance" diagram at the bottom of this page).
Some higher end and more expensive models like the Kohler Power Lite use a small electric pump to boost flushing pressure instead of compressed air. Expect to pay 2 to 3 times as much for these electric powered versions as you'd pay for a compressed air model.