A little more explanation might help clear up what he means.
"The axis of pivot for a straight-handled ax lies in the center of the handle throughout its entire length from end knob to top side of the eye.
But with the curved handle, any rotation is controlled by the chopper’s hands grasping the lower curve at the grip. Therefore the real axis of pivot does not pass through the ax head at all because the 10 degree bend of the lower handgrip is not pointed in that direction. The effective and real axis lies in an extension of the grip and passes somewhere to the rear of the entire axhead.
"The trouble-making grip with its 10 degree bend subtends an arc of 4½” at the handle's residual length of 25". That means that the curved-handle axe already has a constructive "foresection" of 4½” behind what we have hitherto termed the “axis of pivot”. But there is also an additional 4½” waiting at that point. Remember, we stated that each ax has a fore-section distance of 4½” forward of the axis of pivot (by the string-suspension method). For curved-handle ax then, the constructive fore-section length between the real axis of pivot and the bit is: 2x4½”, or 9”.
"So the ax with a curved handle will act as if it had an imposing bit 9” long. For a rotation of only 5 degrees, its bit will swing .78”, exactly twice what that same ax would deviate if hung on a straight handle. Greater rotation would bring greater deviation in the same proportion. This is the most damning case against the curved handle. It is substantially less accurate than a straight handle.
"However, the human body is a marvelous machine. It can adapt to nearly anything. If the handle of a golf club were shaped like a pretzel, some people would still play golf. A chopper soon adapts himself to a curved ax handle even though that handle is designed to frustrate accuracy. The chopper acts as if there were a straight axis throughout the entire length of the handle, even though there is not.
"But this unconscious adaptation has a price. Use of the curved handle requires more practice to cut well. And even with practice, the chopper cannot attain the results possible with a straight handle. In our history, this is at least partially confirmed by the woods professionals of the era when trees were felled with axes. These men graduated from single-bitted axes on straight handles to double-bitted axes, also on straight handles. Curved ax handles do not seem to have ever attained widespread professional acceptance."