The tendency for thin-face irons to sound clacky or tinny is a challenge for designers. Fillers behind the face or in the cavity damp sound but can come at the expense of ball speed. Callaway uses a urethane compound impregnated with millions of microscopic spheres of air. The material is positioned in the lower half of the iron where impacts occur, providing the necessary cushy feel but still allowing the face to flex.
Reminds me of the old Bertha irons. The tech makes it easy to get in the air. They launch really high.
Very forgiving. Mis-hit shots still go the distance you expect.