M50: You put the thumb closed against the side of the index finger. If you hold your palms toward you when you grab the rod, the index finger is up and the little finger is down. The thumb is not involved in gripping the rod at all.
The hand is used to holding rope or poles, or braches where the palms are facing you as you pull. The strength of the hand comes from the middle, ring, and little fingers closing around the rope in support of the larger, and usually strong index finger. The thumb assists all the fingers in holding on.
When you turn the palms so they are away from you, and then turn them so the thumbs would point down if if you move them away from their position against the side of the index finger, the had has no normal grip strength to hold onto something that is going through the fingers. In fact, its difficult to use enough grip strength with this hand position to push the rod down the barrel, as you are working against the normal muscles of the hand as they grip anything palm facing you. When you eliminate the thumb's support and muscle structure from gripping a rod, using only the four fingers wrapped around it, and then push AGAINST the normal muscles suddenly, the brain says " Let GO!" to the muscles and instantly relaxes the muscles in the fingers, allowing them to open. In fact, this reaction is almost faster than can be recorded on high speed photograph, because the panic, and pain triggers within the nervous system are so well established, all to protect the body from injury. The fact that the rod would be closer to the face only adds the reaction of the eyes and ears to trigger the muscles to relax and open the fingers up to let the rod pass before causing injury.
This does not mean that injuries do not occur. The explosive force of powder going off under a ramrod is as sudden as it would be behind a bullet, but the ramrod, being much, much heavier, has greater inertia, and seals the barrel from the expanding gases far less, so that it is going to move slower than a bullet or ball, and it takes much more time for the ramrod to gain speed when its also being held by your hands. But, certainly skin friction burns can be expected on one or both hands before the fingers release the rod to let it go by. If the back of the lowest hand is near the muzzle, it is most likely going to receive burns from the powder, too. I have had severe rope burns from ropes being jerked from my hands suddenly, and the skin was turned into a wax like substance that pushed off the hand and fingers as if it were melted wax and not flesh. ( I still have the scars).
So, my recommendation of using the " monkey grip " is my solution to loading a PRB down the barrel with minimum exposure of your hands and fingers to danger in the event of an unexpected discharge. If you watch how Chimpansees and other monkeys move about tree branches, they grab the branches with the 4 fingers, and not the thumb, which is almost never used for that purpose. When they move quickly throug the trees swinging and grabbing and releasing one vine or branch after another, using the thumb to close around a vine or branch would be dangerous, not only causing injury to the hands, but causing a serious fall.
All I have done is look at how the reacts to different pressures from different directions, and watched how the closing of the hand allows us to hold onto something. The Thumb is what we use to keep something from pushing out the top of our hands, between the thumb and forefinger. Slide down a pole with and without using your thumb and see how easy or difficult it is to remain on the pole. Then try the same stunt upside down, if you can do it. Go ahead and use your legs to help hold your body to the pole. No matter how much pressure you put on the thumb to close around the pole, you just don't have the strength or leverage to hold on the pole that you do when you are right side up.