http://www.fuzzyyellowballs.com/improve-your-forehand-in-45-minutes/

The Windshield Wiper Forehand is a variation of the classic tennis forehand, and it is the "go-to" stroke for many pros on tour today. The key to learning the Windshield Wiper is to alter the way you swing up at the tennis ball and then follow through – not just tack a winshield-wiper finish onto classic forehand mechanics.