The "auction" algorithm
Upper rods are falling, contacts are made, and finally the third lower rod gets overloaded by the first and the fourth upper rods. The whole group of rods including the overloaded and the overloading ones is marked in yellow. Note also that the second lower rod is underloaded: no upper rod touches it. The initialization stage of the auction algorithm is thus finished, and we begin redistributing upper rods between lower ones.
In the next slide we will let the yellow group of rods go down while keeping the rest of lower rods at their positions and see how one of the overloading rods catches another lower rod and stays behind.