There are two separate effects. First, higher CYA levels mean less total chlorine used, so you can use a smaller SWG and it will last longer than it would at a lower CYA level. Second, SWGs somehow reduce several of the bad side effects of higher CYA levels that are normally experienced with other forms of chlorine. So the higher levels don't have any more problems than a lower CYA level without a SWG. No one really seems to know why that is, but it is probably due to the extremely high FC levels inside the SWG cell.