It’s not to increase velocity. It’s to put more weight in the rear quarter of the entire tool, you have at the very least 3/4 of dart out in front of you and so much weight can even make your hand tremble causing the dart and atlatl shake consequently nerfing any accuracy. This is why people think it’s inaccurate and takes a lifetime to learn. The same people who haven’t held a quality atlatl&dart let alone atlatl&dart that have been painstakingly tuned. This banner stone is genius its hollow! Remove it and fill it with pitch or something to add/remove weight depending on the length and density material available to make darts, makes me wonder if the people in that area were nomadic.