The ERC II features an ultra thin, forged titanium face with patented Variable Face Thickness Technology - delivering higher ball speed off the clubface and exceptional forgiveness. The ERC II does not conform to USGA rules.
One of the tests that the USGA do is to fire a golf ball at a stationary club head at a specific speed. When the ball strikes the face of the club it bounces back at a certain speed. Those two speeds are compared to determine the head's efficiency. If the output speed is more than 83% of the input speed, the club fails the test and is deemed non-conforming.
The idea behind the club head design on the erc II is to create a spring like effect when the ball strikes the face of the club, helping you to propell the ball in the direction that you want to. Many people find that this club helps them and many find that it doesn't. To find out for yourself go out and hit a Callaway erc II.