The spring and rubber jaws grip the glass, automatically freeing fingers and wrist to do faster and more accurate projects.  The glass is held firm by the spring pressure, allowing more force into the grinding wheel, thereby speeding up the grinding operation.  A curcular pivot base makes it easier to generate a non-straight configuration on the edge of the glass, wiothout snagging on obstructions on the surface of the table (such as loose galss splinters).