With the advent of flight and the increasing technology there involved, airplanes became faster and faster. After some years, many scientists and engineers began to imagine that the plane might be designed that could break the sound barrier; which is to say that an airplane might go faster than the speed of sound.
However, try as they might, no one managed to design or pilot a vehicle that survived an encounter with the speed of sound, and far too many planes were destroyed for no apparent reason as they approached the sound barrier.

The answer to the riddle came in the understanding of the nature of sound. Sound travels in perfectly spherecal waves from the origin of the sound. One at a time, these waves are harmless, however all together they create an impressive wall... one which could crash an airplane. A plane that approaches the speed of sound not only has to pass the waves that it has already created, but is continually creating new waves which it must then pass through, and the waves build up quickly.

The answer, then, is not to slowly pass through the barrier... trying to gently break it down, but to punch through swiftly before the waves have a chance to accumulate.
It must be a sudden change in speed, or the craft is destroyed.