The Swiss method entails heating up the sugar and egg whites over a double boiler before whipping the meringue. And once the meringue has been whipped to stiff peaks, you add the dry ingredients and fold them together.
1- The addition of heat to the mixture (when whisking the egg whites and sugar together over the double boiler) helps the proteins unfold more easily during the denaturation process. 2- The melted sugar increases the viscosity of the syrup that coats the air and water particles that constitute the meringue, making a stronger layer of protection