Some materials, while still in the solid form, can take on two different arrangements of the atoms (they exist in two different solid phases.) The two NiTi rods that you dropped are examples of a material that has two different solid phases, depending on temperature and pressure. Because the atoms are arranged differently, the two different solid phases of NiTi sound different when they are dropped. The arrow indicates the front of the phase change as it travels through the material, converting it from one phase to another.