The salt is part of the process I'm not too sure on why though..i think it maybe something to do with opening the 'paws' for want of a better word of the nylon in the wheels to allow the dye to penetrate properly (Like I say I'm not too sure but there will be a expert that could correct me)...
Also the water is at boiling point as well...the hotter the water the better the result...pop the wheel in,bring the water to boiling point then turn down the heat and let it simmer for a good few hours (this is what I did anyway)