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Um.....it controls the idle. Like, if you want it to warm up faster in the winter or kick it up to keep the A/C cool in the summer. You hit the button and the rpm ramps up make that possible.
Can you use it.....of course you can. I don't have an APCM, but I do have a high idle setting on my chip which I use fairly regularly. Do you need it....probably not. Unless you spend extended time idling in extreme cold weather it's just another fun little gadget.
It also has charge protect, where it will automatically adjust the RPM to maintain the alternator output at 14.5 volt, which is really designed for something like an ambulance when it is idleing with the all the lights on, etc.
Some also have a PTO switch built in, which when engaged will close a set of internal contacts designed to engage an electric clutch, while at the same time adjusting the engine RPM to a preprogrammed speed.
It's a fun gadget, but very expensive if you buy it new. I picked a new take off (removed from a new cab/chassis unit at an ambulance mfg co) up at a garage sale for $25. Otherwise I wouldn't have one either.
Oh, and it is also a 4 digit, digital tachometer as the default display, reading down to 1 RPM. This is great for the van, as it doesn't have a factory tach.