those lines are the hit zone. .... think of it as your margin of error. The longer you hold at the top of your swing ( holding the mouse) , the smaller your margin of error gets, just like real golf. That said, it really doesn't matter much ... if you miss the perfect line, your shot will go left or right anyway, but the closer to the edges of the hit zone, the more errant the shot is... and if you shrink it down, that slight mishit becomes huge.
Jeff