Throughout the descent of a bench, the lats shorten since the shoulder is experiencing extension and slight horizontal abduction. During the ascent, the lats are stretched a bit when the movement is reversed — this shows that they aren’t contracting during the ascent to “help the bar up”. However, since the shoulder maintains external rotation, the lats maintain a steady stretch, or constant tension, throughout the descent and ascent. When the lat maintains it’s tension it reinforces the shoulder and makes it incredibly more stable. Tension and stability equals more force application in the bench.