As long as it's on, it's always working. It's an Angle of Attack indicator. 1 green bar at the bottom means optimal angle of attack. As soon as you start increasing AoA (slowing down, while maintaining altitude), you'll get all indicators, except the bottom one. This is telling you your AoA has slightly increased. As you continue to slow down, while maintaining altitude, the green bars slowly disappear into the yellow. If you keep slowing down, and don't lower the nose, the bars will keep disappearing. Once you are in the red, a stall is about to happen.
Remember, the bars DISAPPEAR as your AoA increases. This is to declutter the display to make it easier to see out the front.
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