...I just understood why a particular trick I use works in terms of call-waiting: okay...when you are on a phone that doesn't have a Link option (to transfer from one line to another) and you have another person ring-in, you know how you can QUICKLY hang up the phone and then lift up off the hook switch and get the person on the other line? The "command" to tell the Central Office to switch lines for you is actually a signal from the hook switch lasting between 200 and 1,200 ms (from roughly one fifth a second to a second and a quarter) - so if you punch the Link button your phone, you're, in effect - spoofing a hang up for roughly that amount of time...

    ....the reason this works is because the hook switch is a signalling device to the central office (when you lift up, the CO knows to send you the dial-tone, and when you push down, it tells the CO to "hang up"). Commands via the hook switch are time-based.