I hope iŽll manage to explain that with my bad english as iŽm not a native speaker but iŽll try...

A swirl pot is mounted to get air bubbles out of your cooling system. When the coolant passes hot spots of the engine, it can partly start boiling and small air or better steam bubbles appear. This will cause your cooling system not to work so efficiently.
If a swirl pot is mounted, those steam bubbles will exit the system there.


I hope i could help you with that explanation...