This is going to sound smarter than I actually am… or make me sound like an idiot.
I think what is happening is it is adding mass to the bow which changes how the bow moves when releasing the energy. An object at rest tends to stay at rest…. more mass is harder to move so energy released when shooting an arrow would want to move the bow, not just the arrow and a little more mass will result in a little less movement helping with clearance and torque during the shock of load releasing an arrow. I have a light grip, likely poor form and torque my bow when I shoot. With this added mass the bow would react less in my hand as I release the arrow causing the bows transfer of energy to the arrow to be cleaner, the arrow clearance on the riser to be less affected by the bows movement, energy from limb tip to the center of the string or knocking point is relative to the energy from limb tip to the center of the riser. When released the mass of the riser pushing back against your hand caused the energy to focus on that knocking point or fingers during the draw as much as the palm of your bow hand, when released loaded energy push/pull (however you want to think about it) the arrow forward releasing the pressure on my bow hand at the same time which will let the riser move, in this moment the riser can do what it wants and moves where the energy tells it to when it is released. With my light grip, torque and poor form I think it is less effected with the added mass. (Welcome to the mind of someone with ADD)
In reference to it making the spine of the arrow seem stiffer (not actually stiffer, just making it act like a stiffer arrow), it would make sense that added mass to the limbs would cause a little slower limb movement although I did not have any change to speed so this one stumped me. It could have slowed the limbs with energy transfer but made up the speed with a cleaner release? If someone could explain that to me I could get back to work instead of staring at the wall after reading
@gcr0003 response… productivity just went out the window for me today until I figure it out.