In theory, it should raise the revs a bit. Used to work well on old school cars, never had the need to try it on newer ones, but I cant see the science of finding a possible air leak having changed over the years. Basically the air leak will, in theory, be allowing unmetered air to be sneaking in after the MAF, therefore causing your rough idling. Spraying the cold start ( which is a flamable liquid ) should help the combustion, therefore raising your revs.