Think of non-progressive as the kind that's staged that switches on at (x) boost based off of a separate pressure sensor. Alot of the WI kits and such are configured like this. They're usually referred to as 'Stages' -- Single stage or dual stage. These are for systems that are pre-pressurized normally that will fire open a solenoid to allow that said-pressure WI/alcohol to spray into the intake plumbing in an abrupt, quick blast. Your garden-variety, cheapy Aux injection systems are like this. I personally don't like these when using alcohol because there's a quick blast of a combustible fuel being introduced in a fairly large quantity into the intake that cannot be altered or adjusted except by changing the nozzle size. As load varies (for example, 6000rpm at 10psi of boost vs 6000rpm at 25psi of boost), the demand for fuel volume to achieve a desired air/fuel ratio will be different. The non-progressive, "staged" setups don't make a compromise of this well at all -- the volume they spray will be too much at too low of a load, just right somewhere in the middle, then way too little as load rises into excess. The general approach to non-progressive configurations is that they're a "turn it on at this boost" and the amount of water/alcohol injection volume is minimal if even measured or calculated at all.
Progressive ones are based off of at the very least load; load in this case being a reading off of a MAP sensor that senses boost. Julio's Alkycontrol kit, for example, borrows the 5V output signal off the GM 3Bar MAP sensor and uses it to adjust pump voltage -- as the pump voltage rises, the pressure in the system rises, therefore adjusting the alcohol volume that's being injected into the intake. This establishes a "load curve" that can be considerably more easier to tune with. There are some fancier, gee-whiz controllers out there that will adjust according to TPS, RPM, Load, etc. but I honestly don't believe these are necessary. Fuel curves for EFI systems typically rise up to torque peak, plateau, then ever-so-steadily drop based off of RPM. In the case of load (boost/manifold absolute pressure), in this case, the fuel delivery curve is pretty much linear. The subtle changes caused by engine RPM and it's VE (volumetric efficiency) can be adjusted via the EFI system underneath where the alcohol "rests above". The paradigm for tuning a progressive system such as Julio's is as a gross fuel replacement -- let's yank 25% of fuel from an already, tuned on 100% fuel only car, and replace it with alcohol. The progressive approach makes it easy to not only tune for that replacement but also achieve solid, target air/fuel ratios throughout the RPM band at varying loads.
Hope that helps,
B
