Gear Effect is assisted by bulge and roll - which is the curvature of the club head expressed as the diameter of a circle in inches. Common measurements of bulge and roll would be 8 to 12 inches.
What happens when you strike a ball out towards the toe of the clubhead, is the head attempts to rotate about it's vertical center of gravity - and this rotation, along with the sliding motion of the golf ball on the face, acts like two meshed gears, imparting spin on the golf ball that will bring it back into the center of the fairway.
A ball struck out toward the toe will spin back toward the middle with a hook motion; while a ball struck toward the heel will work back toward the middle with a slice action.
How do bulge and toll play their parts? You might be interested in this article by Rod Cross, from Sydney University published in July of 2006 called The Physics of Golf. In this article Rod details the interaction of two sliding surfaces as it relates to the golf ball and the club head in lay terms.
For those wishing to delver deeper into the mysteries of bulge and roll, a patent was issued in 1946 to Baymiller and Vose who were working for A.G. Spalding covering the addition of bulge and roll to a golf club. It's attached to this blog entry and I hope you take the time to read through it - it's very interesting indeed.
In addition to the technical description in the patent, John W. Baymiller had an article published in the July 1965 issue of Golf Digest, titled Straighter Shots from Curved Clubfaces - it too is attached to this article compliments of Bernie Baymiller. Enjoy!
Tim Hewitt, myOstrich Golf
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