It was Head's swing style marketing scheme in the late 90's to compete with Wilson's x.x numbering system or Prince's power index.
As you go down the L's and back up the S's the racquets get lighter, stiffer, and more powerful. They still have some racquets left from the Titanium S line (S1, S2, S6, S16, etc) and yes, an S16 or an S10 is so light, big, and powerful that it does seem to basically play by itself. Oy.