[O.B.Left]

Thanks for explanation. Now I understand the "out" part. But it leads me to one more question: why is the on-plane swing not "cross-line" at the impact point?

Thanks,
DK

It is in a manner of speaking. Although In TGM speak "Cross Line" is reserved for a different consideration, more on that later. To your point about straight away ball flight, for balls addressed back of low point the clubhead will travel an arc (assuming arc of approach method) that extends down and out to low point. Post low point it will travel a similar arc up and in. Interestingly the divot will look straight at the hole. Although I have noticed that if you turn the divot over you can see a little arc to it. If you were to swing without this Down Out and Forward , Arc of Approach and instead "steer" your clubhead straight towards the hole , your divot would point left of the hole. This sadly, is by far the most common divot direction and no doubt due to false logic about swinging straight at the hole. See "steering" when you buy your book.

Oh ya, both "on line" and "cross line" are On Plane really, just different planes. For straight away ball flight, "on-line" despite the Out, Down, Forward you still trace (point at) a straight Plane Line. Whereas for curved flight you will construct a different plane, Cross Line to the original. A new Plane for you to travel, with a new Plane Line for you to "trace" and having its own associated Out, Down , Forward Club Head Orbit , Three Dimensional Impact (which is fantastic to feel).