Used by manufacturers to refer to the springlike effect of a flexing clubface. The U.S. Golf Association limit is .83, which refers to the efficiency of energy transfer between two colliding bodies (clubhead and ball). The way a thin-titanium clubface flexes can reduce the way the ball deforms at impact, therefore decreasing the loss of energy transferred to the ball.
LEADING EDGE
The forward edge of the sole, opposite the trailing edge.
MOMENT OF INERTIA (MOI)
The measure of a club's resistance to twisting on off-center hits (sometimes called forgiveness). The USGA limit for drivers is 5,900 grams/centimeters squared. Golf Digest, with help from David Lee, associate professor at Gordon College, tested the MOI ratings for each driver that made our Hot List, using one sample head from each club. We then sorted the drivers into three groups. Drivers labeled "HIGH" had an MOI test number from 4,000 to 4,600. Drivers labeled "VERY HIGH" had a test number from 4,700 to 5,300. Drivers labeled "EXTREME" had a test number higher than 5,500. The Nike Sumo2 5900 had the highest MOI number of any driver we tested.
RELIEF
The angling or curve up and back of the sole to reduce turf contact.
SKIRT
The perimeter of a clubhead between the sole and crown.
