All DSMs and the EVO VIII/IX include a front O2 sensor. The front O2 sensor is an important part of the stock ECU's feedback mechanism, as the stock ECU uses the front O2 sensor to know how to adjust fuel at idle and low/mid RPM cruising.
The ECU+ includes a feature that allows you to use a wideband O2 sensor kit (supported in the ECU+ for datalogging air-fuel ratio) and toss out your front O2 sensor.
Why would you want to do this? Simple - if you remove your front O2 sensor, you can install a wideband O2 sensor in its place. The front O2 sensor is in the downpipe in an optimal place to install most wideband sensors. With the ECU+, you can install your wideband sensor without removing the downpipe and having a new wideband sensor "bung" installed. Instead, just unscrew the front O2 sensor and screw in the wideband sensor.
How's it work? With the ECU+, you select what kind of wideband kit you have, and the ECU+ will convert the wideband O2 voltage (which is not at all usable as a front O2 sensor voltage) into the equivalent front O2 voltage - exactly what the stock ECU expects to see. The ECU+ does the work, so you don't have to!
One other cool benefit of this feature is that you can set the air-fuel ratio that the stock ECU will home in on at idle and cruise. By default, the front O2 sensor will cause the stock ECU to target 14.7 AFR at idle and cruise. With the ECU+, you can run leaner (to save gas) or richer (if your car runs better that way) than stock. This is something that just can't be done without using a hardware engine management system like the ECU+.
All 2nd generation DSMs and the EVO VIII/IX include a rear O2 sensor. This sensor is mounted just behind the catalytic converter, and is used by the stock ECU to verify that a cat is installed and working properly. If the stock ECU senses that the cat has been removed or replaced with a high flow model, it will often generat a check-engine light indicating a "catalyst inefficiency" code. This code is annoying and can cause your vehicle to fail OBD-II emissions tests.
To correct this problem, the ECU+ includes a hardware rear O2 simulator. This hardware outputs a signal which is very close to that produced by the rear O2 sensor when a catalytic converter is installed. As such, it fools the stock ECU into thinking that there's no problem, cat-wise, and thus the stock ECU never generates the error code.
This feature can, of course, be used at the same time as the front O2 simulator, above.