Prime Shift

Prime Shift is an ability exclusive to the Spectre faction and its variants. When the Barricade Shield (or any variant) is equipped, an extra meter is shown above the health bar in combat. This meter fills up when in active combat, but otherwise drains away. When the meter fills up enough to enter the highlighted region, the bar changes from white to yellow and Prime Shift is initiated, which is a precondition to various Overdrives.

Explanation
The Prime Shift meter is completely empty at the beginning of each combat. There are several ways to charge it up:
 * Dealing health damage to an enemy ship or powered module charges the bar according to the Attack Charge Rate. Exception: Damage dealt to Altairian Pylon Emitters is never counted.
 * Taking health damage charges the bar according to the Defense Charge Rate.

Prime Shift begins while the meter is above the Prime Shift Threshold and one second thereafter.

If the precondition is not again met before the Charge Duration expires, it ceases to charge. Afterwards, if Prime Shift does not gain any charge for the period indicated by the Drain Delay stat, the meter begins to drain according to the Drain Rate.

Trivia

 * Inferring from the Barricade Shield's in-game description, Prime Shift Charge is a representation of combat experience.