The knives seemed to be pulled by strings to me seeing their shape with hook-handles. The movement however was a straight line, so maybe a whip-like movement or magnets to shoot them?
It can’t be something set-up around him as he walks around with them. It sounds like his arm/sleeve has strings to it and his grand movement “shoots” the knives like a slingshot.
EDIT: seeing the knives design closer, there are two hooks to them. This can explain how they are shot with the blade in front, as the butt of the knife being held by the string keeps it in position during flight, while the smaller hook being pulled would shoot it up. If his sleeves and fingers have strings attached, the arm movement would throw the knives forward with the first string while the second could be used to keep the position and potentially pull the knives back