It grants the capability to perform two combat maneuvers for each time the Hit Streak is charged.
When unlocked, the first maneuver that expends a charge (such as Execution) will not cause the Hit Streak number to stop glowing red. Only the second use will turn the Hit Streak number white again.
This ability is extremely beneficial if one prefers to take down Defenders or Berserkers very quickly in combat.
Note that after the first charge has been used, the player cannot raise it back to two charges by building the counter. Talion must expend the second charge, and then build the counter by another 8 (or 5 with Blade Master), to go back to a full double charge.