You gain 5% more of the Haste stat from all sources, and your off-hand attacks have a chance to generate 15 Energy. Slower weapons have a higher chance.
Combat Potency is a passive rogue ability learned at level 30 for those with the Combat specialization. While giving a small buff to haste from all sources, it gives successful off-hand attacks a chance to generate energy. Slower weapons have a higher chance.
Patch 3.0.3 (04-Nov-2008): Now only works with auto attacks (no more Shiv.)
Patch 2.0.7 (13-Feb-2007): The talent "Combat Potency" no longer causes rogues to have a chance to gain energy when other players hit them with off-hand attacks.