Hi All,
Just my piece in here.
Vertical pip structure allows better speed generation for shakehand movement as u tend to swing above ur wrist. (sorry, dont know how to explain this better, but think of ur FH, where the head of the blade is pointing away from ur body and swinging upwards, hence, the vertical pip forms a line to support the movement)
Horizontal pip stricture allows better speed generation for Pen grip - as compare to shakehand, pengrip swing below the wrist (try to think of Ma Lin, when doing FH loop, the blade head is pointing towards downward-right and then swing up; hence the horizontal pip forms a line and supports the movement better)
Lastly, Horizontal pip is more consistent in flat ht bcoz hit tend to be less upward movement but mostly forward, as which horizon pip provides the necessary support - thats why majority of short pip are horizontal structure even in shakehand. This could also be explained as vertical pip for hitting can soemtimes generate too much kick from the pip hence make the hit inconsistent.
Cheers,
H