when particle A collides with particle B in waveform mode, they combine into a particle C with the sum of their current movement directions. (e.g. particle A moving upward hits particle B moving left, results in particle C moving up and left)
when particle C collides with another particle C, they split into two A's and two B's each with one of the component directions of the C's. (e.g. one C moving down and right hits another C moving down and left, the result is a B moving left, a B moving right, and two A's moving down.