Let us consider this with two symmetrical pulses traveling in opposite directions; they are exactly alike except that one is positive & other negative. As they pass through each other, there comes a moment at which the whole string is straight. Where did the energy go during the annihilation?

After some time, the pulses reappear. But "what is it that preserves the memory of them through the stage of zero displacement, so that they are recovered again in their original form"? It is the velocity of the different parts of the system. The string at the instant of zero deformation has a distribution of transverse deformation has a distribution of transverse velocities characteristic of the two superposed pulses. Transverse displacements cancel, but transverse velocities add, and for this one instant the whole energy of the system resides in the kinetic energy associated with these velocities.