This use of centrifugal force is especially effective for smaller brushes.
This tip works especially well for cleaning small brushes. Run an oversize screw into the paintbrush at approximately its center of gravity. Then tighten the point of the screw into the chuck of your drill/driver. Now dip it in some thinner, and spin it in a laundry sink or 5-gal. bucket. Repeat this a few times to fully clean the brush.
— Ray Berntsen, Seattle