Shane there could be a number of reasons why he tells you to hit 100yds. Too many to list so I won't. But the left wrist idea is some what true or should I say a piece of a legit theory. If you strike the ball, at impact, with the back of you left hand your club face will be on the proper path through the ball. The toe will be turning down and through the ball (hence a slight flex in the left wrist) if the swing continues through the ball the toe will turn over and give you right to left spin, provided the club is on the proper path.