The ECU+ can datalog knock using either its native logging of the sensor's knock voltage, or as the stock ECU's version, called knock sum.
For knock sum, the ECU+ logs knock through its MUT-II logging abilities. Knock sum is a simple number calculated inside the stock ECU which ranges from 0 to 20 or so. Higher numbers indicate more knock, and generally you'll want to keep knock sum below 7 or 8.
Knock voltage is the raw voltage from the knock sensor, after the ECU+ has filtered and conditioned the signal. (Note: most engine management systems that log knock voltage don't do filtering and conditioning, and end up with an inaccurate knock voltage value. Filtering and conditioning removes extraneous out-of-band voltages and only looks at the knock voltage in the region of ignition.) With knock voltage, you're looking for extra large spikes to indicate real knock.
The ECU+ gives you the best of both worlds, with knock sum for most tuning, and knock voltage to see a faster, more precise picture of what's going on.
Why use knock voltage instead of knock sum for back off?
One word: speed.
The ECU+'s internal circuitry examines the knock voltage continuously. That means that at the instant that knock is sensed, the ECU+ can implement the knock back-off. With knock sum, the ECU+ would need to wait until the stock ECU noticed the knock, updated its knock sum value, and sent the datalogged value to the ECU+. In the grand scheme of things, knock sum logging is slow - perhaps 10 samples per second. At 9000 RPM, ignition events are occurring 300 times per second. With a knock sum-based back-off, 30 ignition firings could occur before the back-off could kick in (and that's the best case). With the ECU+'s internal voltage-based back-off, the back-off is implemented in time for the next firing of the engine.
The ECU+ includes a safety measure that allows you to back off timing and run richer when the ECU+ notices a high knock voltage.
With the ECU+, you just set a target knock voltage, as well as a fuel addition and timing subtraction, and let the ECU+ do the work. As soon as the system notices a high knock value, these corrections will be applied to your current fuel and timing maps to sqeulch the knock. This happens immediately after the ECU+ notices the knock, thus protecting your engine.