A downhole tool used in conjunction with a jar to store energy that is suddenly released when the jar is activated. The energy provides an impact force that operates associated downhole tools or, in a contingency role, helps release a tool string that has become stuck. Depending on the operating mode, the energy in tension or compression can be stored by means of a mechanical spring or a compressible fluid such as nitrogen gas. Accelerators should be selected on the basis of their compatibility with the jar to be used.