For example, in Windows you need to edit a Chrome shortcut: right-click on the shortcut, click on "Properties" and append a space followed by the flags above to the target field.
I tried this feature using the latest Dev Channel build for Windows and a recent Chromium build. Most changes are synchronized almost instantly, but not everything is synchronized: Greasemonkey scripts are ignored and uninstalling extensions doesn't propagate to other clients.
It's important to keep in mind that this is an initial implementation of the feature and that Chrome Dev Channel releases are buggier and less reliable than the stable releases.
Chrome Dev Channel and Chromium, using the same extensions