Ka Guard. What exactly is it?

I have seen J outs with it on, and no false Ka alerts that I can remember.

I have seen absolutely zero J outs with it off and a gazillion Ka falses.

Does it simply just change the sensitivity for Ka or is there more to it? I have seen outstanding results with it on and off and come to the conclusion(at least until you guys shed some light on the situation if you would be so kind) that in urban areas on is great and rural areas off is great. Thanks