Goosey
.
Will be trying both methods in about an hours time.
But just thinking out loud, I would think lying across the bench with an arch would make you more inclind to incorperate your lats, legs and core?
Kai in his video made a point that you should be flat along the bench and not lifting your core or be swinging your shoulders/back into the motion.
Kai makes sense.
But either way (under heavy load)there is going to be flexion, which is a good thing (for the spine)as there is no direct loading on the spine in the fully stretched position of the rep.