Solar Gauntlet Overcoat is an enhanced version of the Solar Gauntlets created by Kai. It is done by, in addition to the standard bodybuilding practises, first making the hands incredibly cold. This creates a survival response in the body where it rushes to send blood to the hands to avoid damage to the tissues.
While initially done with cold water, Kai eventually moves up to using dry ice for the technique. Ken comments that his skin is probably frostbitten as a result.
Unlike the Solar Gauntlets which, like Solar Hands, makes the hands warm all the time, the Overcoat is a temporary technique that is done directly before kneading the dough, and it would be dangerous to try and have them all the time.