I had always understood mode S to be used in TCAS equipped aircraft which allows two systems to 'talk' to each other so that if resolution advisories are given they do not conflict - i.e one to climb, the other to descend, but never to give advisories in the same sense such as for both to climb.
When first learning to fly I, I was advised by different instructors to either switch to standby whilst changing, or to dial it straight in as there is a delay built into the system. However, as all this was in light aircraft I'm still none the wiser. Best advice I did get I suppose was to avoid dialling through '7' altogether unless it is in the squawk.
