I take a stab at it.The esab setup make the little welder think its alot bigger on thick aluminum.I think it's a good idea for that machine.It's setup for hi freq at starting,and when you step on the pedal it changes to a lower freq.that make the puddle more fluid faster.So that is where you would use 20 hz,thicker aluminum.For higher freq it narrows the arc cone down to get in the center of a fillet weld.It take more juice to get the puddle wet,so you really need to step on the power.Also might be helpful on real thin stuff,maybe.I have a Thermal 300 prowave,and find 150 hz plenty.I never really uses it there,mostly around 80 to 90 hz.On the wave balance it's a non issue,I run my wave balance about 25 to 30% eletrode negative.I think the miller means 30% electrode positive,and 90% electrode negative.So both the miller,and lincoln do the 90% in the right direction.My Thermalarc goes to 95% electrode negative.That doesn't really mean a thing.Hope some of this helped.
I have an ESAB Heliarc 161 AC/DC, and I'll try to answer at least one of your questions. In AC mode (presumably welding aluminum), as you decrease the power with the pedal, the frequency goes up; increase the power with the pedal, and the frequency goes down. I believe it varies between 20 and 200Hz, if I remember correctly.
I've never gotten an explanation of why it works this way, but based on the 1st reply, I believe this makes sense. As you increase the power/decrease the frequency, the puddle forms quicker (which is what you'd of course be trying to accomplish by increasing the power), and vice versa.
HTH,
Dave