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"In my experience, if you stay with the essentials you WILL build a repeatable swing undoubtedly. If you can master the Imperatives you have a champion" (Vikram).
The reason you can't sustain the lag is because you are so eager to make the club move fast (a reaction to the intent of "hitting it far"). So on a full shot you throw it away too early, which doesn't happen for your short chip. (bts)
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When James Durham recorded 94 at the Old Course at St Andrews in 1767, he set a course record that lasted 86 years. Golf: A curious sport whose object is to put a very small ball in a very small hole with implements ill desiged for the purpose - Sir Winston Churchill