It controls arc current as a function of voltage, or how much the machine will increase the current for any drop in arc voltage. The arc voltage is determined primarily by the gap between the electrode and the work, and what elements makeup the plasma (the arc). With arc force at zero, the machine should produce the same current regardless of arc voltage (gap); of course, within the machine's electrical limits. With arc force greater than zero, the machine will decrease the current as the arc voltage (gap) increases; the rate of increase being dependent on the knob setting, and again the machine's electrical limits.

Here is a lengthy explanation from miller.
http://www.millerwelds.com/resources...rticle108.html
Mark gives an answer in another thread
http://www.everlastgenerators.com/fo...t-is-arc-Force
Ben Krasnow has a comment also.
http://benkrasnow.blogspot.com/2009/...r-welding.html

As a general rule, the settings should be at whatever position you find it easiest to produce an optimum weld.