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 Constructive Interference 

 Constructive interference occurs whenever waves come together so that they are in phase with each other. This means that their oscillations at a given point are in the same direction, the resulting amplitude at that point being much larger than the amplitude of an individual wave. For two waves of equal amplitude interfering constructively, the resulting amplitude is twice as large as the amplitude of an individual wave. For 100 waves of the same amplitude interfering constructively, the resulting amplitude is 100 times larger than the amplitude of an individual wave. Constructive interference, then, can produce a significant increase in amplitude.

 Definition and Graphic Provided by Boston University

In the video, it appears as two waves approach each other then are reflected back when they collide.  What is really happening is the two waves approach each other, when the collide, the waves interfere with each other constructively.  The two waves and during the interference add to each other and the amplitudes grow.