here's what I think
First of all, there is a threshold. I know, in my mothers Bonneville its 5 MPH. I've always heard ABS cheks all 4 wheels to see if any of them are traveling at drasticly different rotation speeds. If one is rotating extremely slower than the others, the system "thinks" the wheel is locked up, and pulses the brakes. So, think....If all four wheels lock up at the same time.... (as on ice) the computer thinks nothing is wrong. So it doesnt pulsalte the brakes, they just lock up! This is just my theory, im not sure, just makes since to me.
"If I teach you half of what I forgot, you'll still have enough to compete with most people"