EFIE - like what SVOboy said, it basically fools you car's computer into reading a false value, letting you manually force the engine rich or lean.
In electrolysis, H2 is produced at the cathode (negative electrode) and O2 at the anode (positive electrode). The gasses mix once they hit the surface so they must be separated in the electrolyte. My idea was to put a sleave (like a tube) around each electrode that would catch all the bubbles inside it. This might slow the reaction down some because there isn't as great of a pathway for charge to flow, but I never tested that idea. I don't think that's much of a problem though since there's other things you can do to draw more current and produce more gas.
Scouring the country for an excellent condition Civic VX
EFIE should be like the apexi crap, but it's not as precise or expensive, just a lying box.
The EFIE works by increasing the voltage of the signal coming from the o2 sensor. The o2 sensor tells the ECU how much o2 is in the exhaust based on a voltage returned. the EFIE will take that amount and compensate (user defined I assume). If your modifcation burns 20% less gas you can use the EFIE to change the signal by 20%. That way the computer does not sense that the car is running lean and then overcompensate by running rich.
The safc is nice b/c you can tune by RPM range. I actually thought of getting one on my car to have my 2500-3500 rpm range (my highway cruising speed) leaned out a bit. With the new VTEC-E engine however I shouldn't have to worry about that.
On my car the safc2 would make a huge difference. My car runs dead rich from about 2900rpm. When I had the problem with the distributor twice I noticed a huge power in the top end. One time I advanced the distributor wrong and it screamed like crazy all the way to redline and chirped the tires when it shifted into another gear. Another time I set my distributor wrong at 13 degrees and my topend was so good. The best fuel economy i get is at 20 degrees, which is stock.
So it looks to me like the hydrogen boost system doesn't separate the oxygen and the hydrogen into 2 different tubes.
Also I know platinum is a good electrode too, but come on, it is so freaking expensive and hard to come by.