A shock pulse is a massive energy discharge from a starship's warp core. Generating such a pulse requires that the pressure inside the core be increased to dangerous levels, and it has the potential to cause a subatomic particle shower that could lead to a chain reaction, destroying the ship.
In 2371, B'Elanna Torres and Harry Kim initiated a shock pulse in an attempt to reverse an implosion ring that had surrounded USS Voyager. However, the pulse only increased the rate of implosion. (VOY: "Twisted")