To to put as simply as I can replaces the entire OEM engine management system (there are also piggyback versions)
Most of the replacements let you tune injector duty cycles, ignition advance/retards, ect.. Some control only the fuel injection part (MegaSquirt) and others do both (Electromotive TEC).
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> a Motec, Autronics, Electromotive, Hal etc unit will come with sensors or in some cases use the factory sensors for things like air flow, manifold pressure, air temp, intake air temp, water temp, crank possition, OXS... then it will do some math and decide how long to fire the injectors for (ie how much fuel to add to the air) and when to spark the air/fuel mix.
Sometimes standalone systems do other cool things like Anti Leg, basicaly fireing the air fuel mix with the exaust valve open to keep the turbo spooled up thus removing any turbo leg.
I run an Autronic standalone in my car. You really need it past a certain point. Eventually in the horsepower game you'll need bigger injectors, or more control over the system, and this is where standalone comes in.
Plus I get some extra goodies: Anti-Lag, 2, Step rev control, Distributorless ignition (coil packs), and two "switches" that can be set by rpm, coolent temp, boost, etc. I use one of them for a shift light, but you can also use it for VVT in 1.8t cars or VTEC in Hondas.
I can control ignition timing throughout the rpm range and set by manifold vacuum/boost, as well as my fuel curve throughout.
Stand-alone management is a necessity when you go beyond the parameters that stock (or chipped) engine management can handle. Stand-alone is designed to handle whatever you can possibly throw at it (timing, VVT, boost, nitrous, whatever) so that you make more power without grenading your engine (hopefully).