Your finishing moves have a 4/8/12/16/20% chance per combo point to restore 25 energy.
Relentless Strikes is a Rogue talent located in the 1st tier of the Subtlety Tree. This talent gives all finishing moves a chance to regenerate an instant 25 energy.
Relentless Strikes is one of the best talents you can buy. Make sure you include this in all your combat and mutilate builds.
It essentially allows certain finishing moves to be used in such a way that your energy acts as though you did not use a finishing move at all. For example, if you use a finishing move that requires 25 energy, then you will have not used any energy at all.
Patch 1.6.0 (2005-07-12): Fixed a bug where the energy gained by Relentless Strikes in some cases was increased beyond the value listed in the tooltip.