It's not really used
There's not much training during the initial school on CWS, probably because nobody uses it. However, it seems to be an attitude hold feature. Whatever attitude you have when you select CWS will be held (as long as you're within tolerance). Once in CWS if you want to change the attitude of the jet you make the change with the control wheel and then CWS holds that new attitude.
The newer jets in the fleet do not have CWS buttons installed. The CMD mode of the auto flight system is so good that there's no reason to employ CWS.