From the manufacturer:
Winsor & Newton Artisan Water Mixable Mediums, Fast Drying improves the flow and speeds the drying of Artisan oil colors, smooths brushwork and increases gloss and transparency. It is excellent for glazing and producing fine detail and is resistant to yellowing. This is a very popular medium as it speeds the drying of oil color by about 50%, allowing further layers to be applied more quickly. The formulation has been improved and this has helped to wet the color better.