Jack Dempsey - Championship fighting

In one of the early chapters of Jack Dempsey's book Championship Fighting; he mentions straight punches, he basically says you shouldnt punch with palm facing down, because the minute you turn your hand over to face that way, the arm begins to loop instead of being straight. This makes sense but would you say this applies, like straight punches that are either vertical fist or slanted between vertical and horizontal? Also if anyone has any techniques for training weight shifting or the falling step? If anyone wants to add anything else feel free