This section is from the book "The Golf Swing, The Ernest Jones Method", by Daryn Hammond. Also available from Amazon: The golf swing, the Ernest Jones method.
It is best to begin by making quite short shots with an iron club - a mid-iron or a mashie: what is known as a quarter-swing or a half-swing (Figs. 24, 25, 27, and 28). It is this movement which forms the essential part of the full swing, and it is because this movement is so often absent from the full swing that the ball is not really hit away, but is merely pushed away by the club (Fig. 30). When this relatively slow and powerless movement is performed, the fingers and hands have failed to dominate the movement as the hands come toward their lowest point in the down-swing. Instead of forcing the club-head from its position behind them to a position in front of them in order that the ball may be hit away in the most definite manner, the hands and fingers have failed to exert themselves at the vital moment; they have exercised no leverage over the club, and the shaft and arms have moved through the lower sector of the swing practically in a straight line. In other words, the action of the hands and fingers, obviously essential in the quarter-swing or half-swing, has been absent, and the hands have performed no function other than that of a strap fastening the club to the arms. The control, the power, and the "touch," which should have been in the hands and fingers, have been lost. The shot at the best can only be second-rate. At the worst . . .

Fig. 45. - Shows "give" of fingers in any flexible movement.

Fig. 46. - Figs. 46 and 47 exemplify again the essential "give" of the fingers.

Fig. 27. - Corollary to Fig. 24.
It is the omission from the full swing of the fundamental action in the short swing that causes the normal driving of the amateur to lack that unmistakable quality of definiteness which distinguishes professional play. The full swing is therefore to be conceived as an enlarged quarter-swing - enlarged solely in order that greater impetus may be imparted to the club-head.
 
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