PowerUp is a ingame tracker for Holy Power (for pallies) and Soul Shards (for locks). The display is very basic and similar to other addons. In the picture on your left you can see what it looks, the above bar is Holy Power and the other one Soul Shards.
The overlay you can see on the Holy Power is a timer guesstimating when a charge of Holy Power will expire. This timer can be disabled if undesired.
There are a few in-game options, accessible either via /powerup|/pu or directly via the Interface Options.
These are the general thoughts behind how the Holy Power decay-timer works..
Observation has shown that a charge expires approximately 10 seconds after you receive it, unless you are in combat. If you are in combat the charge persists (unless you use it of course) and when you exit combat a timer will start. There's one very important aspect to make note off though. If you had a charge before you entered combat, with for example 5 seconds left on its duration, and you entered combat and didn't use the charge, the timer will pick up where it paused prior to combat.
Due to latency both between one self and the servers as well as difference when the game considers you out of combat to when you're actually out of combat (and Blizzards internal timer starts) it's not possible to make it 100% accurate. As such there's a flat .25 added onto the timer to account for this delay, but this number can be tweaked if wanted.
Please report bugs if you find them or any other thoughts you might have.
Changes that reflect changes done to combat log events.
Updated for 4.1.
Stable enough to not be beta anymore!