Many players believe the right arm should be straight at impact, this is incorrect. The right arm should be straightening, but not yet straight.
The right arm should only be straight at a point after impact, usually when the clubhead is about 1 foot past where the ball was.
To learn how to do these, we teach our pupils a drill where we place a tee peg in the ground in front of the ball on the correct path and have them feel like they are swinging down and out towards this tee.
Start of doing this with short and slow practice swings, before gradually building up to hitting full shots with a ball. Try also putting the ball on a tee, as we want to concentrate on the swing at this point and not worry about the contact with the ball.
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Filmed at Troon.