I have used them before from Marstons / Wychwood. they do work a treat. Basically its a bright racked cask usually by crash cooling the beer then its either repitched with yeast or in the case of fast cask contains a 'ball' that holds the yeast and is dispersed in to the bright rack to maintain the 'real ale' concept. Fullers also do this with all their cask beer. Its not that uncommon. Makes for a more consistent product. They did experiment with + and - finings at some point so they could be in at as well. I could never get the ball thing out of the cask for closer inspection!
You need to let your cask adjust to cellar temperatures anyway which takes 24 hours as it rarely arrives on the dray at the correct temperature. You would be surprised the percentage of casks you can stillage and serve in an hour (with a hop filter). The key to this is getting a fresh cask from the brewery so the finings arent totally buggered by the time it gets to you!
D