Tuning FX impact hammer spring adjustment - what is it?

It does the same thing. It compress the hammer springs. The wheel have some notch, as you turn higher in the number wheel, it compress the spring more and more. Thats how you get more power. On mk1 and mk2, you use a hex key to turn on the nut left of the wheel, as you increase it will also compress the spring more and more. Giving you more power. On m3, they change it a dial, so you turn the dial to increase tension on the spring. There is a limit on how much you can compress the spring with both the spring adjustment and the wheel. Too much and the gun won't cock.
 
Power wheel is kind of a quick adjustment after setting the spring with the set screws and then use the power wheel to adjust from max speed which could be 900 and to a min speed which could be 745 so it gives you a range of quick adjustment on the hammer spring but you would set the Allen screw on mk2 first and then use the power wheel to adjust the hammer spring they both adjust hammer spring m3 switched to another wheel to turn for both hammer spring adjustments