The disclosed jarring device generates two jarring impacts at the end points of a reciprocating hammer assembly. Initially, flow of pressurized fluid through the jarring device is obstructed by a deformable member. The resulting increase in fluid pressure upstream of the deformable member causes compression of a spring and downstream movement of the hammer assembly to generate a first jarring impact. A further increase in fluid pressure beyond a threshold, causes a release of the obstruction by either deforming the member or by slicing it by pushing it through a slicer. Releasing of the obstruction causes decompression of the spring, and upstream sliding of the hammer assembly to generate a second jarring impact.