Ah! I misunderstood—comes from not sleeping (wish I could say it was because I was working on styrene!) I thought he was using a mix of Future thinned with isopropyl. That might work, but it would have to be carefully applied to small areas.
I have also added a drop or two of alcohol to water washes when I wanted them to bite into the painted surface more. I usually don't use soap in these. Less of the wash will wipe off—good for when you want a more general modification of color over a broad area. If you want to try this, it is better to test it on a scrap of painted plastic first to make sure you are getting the effect you want. As I said, more of such a wash will remain on the surface.