Because the slot has a forward rake but nose of the hand is going vertical. The attached spring is being compressed because of the closing angle.
There's quite a bit of tension against the ratchet at all times. The spring compression from cylinder rotation (1/2 cock) is considerable.
View attachment 378854
View attachment 378855
It's clear considerable movement of the hand happens as the cylinder is rotated. With added tension as the cycle continues the cylinder would have to gain quite a bit of energy to overcome the hand.
This is why with the added weight of ammo on board, it makes it more difficult to manage as the cylinder gets more and more "out of ballance" with each shot fired.
Therefore, the "throw-by" test is done with 2 dummy cartridges or lead slugs to "off balance" the cylinder. The hand spring tension should be enough to not allow throw-by. It's described on page 226 of the Kuhnhausen book.
Mike