Full Impulse is a special throttle mode which gives a massive boost to sublight speed. Full Impulse diverts all ship's power to the Engine Subsystem. Your power settings go to 5/5/100/5. When you exit Full Impulse, your Subsystems power levels have to build back up to their nominal levels at the Power Transfer Rate. At the default Power Transfer Rate, it will take 24 second for a subsystem to build from power level 5 to power level 125
Something to bear in mind. If you are a run and gun type that Full Impulse's into weapon range and open's fire, you aren't going to be very effective. Your Energy Weapons and Shields are going to be barely functional at power level 5. You effectively are shining a flashlight on the target and your Shields are paper thin with no regneration.