Cover lifters work by lifting and supporting the spa cover by it’s centre hinge. Few insulating covers are truly designed to have their entire weight supported by the hinge and most spa cover warranties even have an exclusion regarding any damage caused by cover lifters.
All spa covers eventually begin to absorb water which increases their weight. Even a cover specifically designed to accommodate a spa cover lift will eventually be damaged from this excess weight. Sold as a pair these make a sensible addition to your existing cover lifter.
The Spa Cover Saver eliminates all stress from the covers hinge yet still fully supports the weight of the cover on the lifter !