A double-drive wheel is free-treadling because there's no friction from a brake band. If I take my foot off the treadle now, it will run for many revolutions before it decides to slow down and stop.
As with all Ashford double-drive wheels this one can now be easily switched between double and single-drive modes.
I converted mine in a lunch-break using only a double-drive flyer and whorl (and bobbins of course). There is a little bit of drilling and hammering involved.
If you're interested, I'll be putting together a kit including instructions. In the mean time, it's pretty easy to figure out how to do it and I do stock the double-drive flyer and the bobbins.
I've used a standard flyer here, but you could just as easily use a sliding-hook flyer kit and benefit from the 30% larger bobbins.
Note that if you're buying a new wheel, the double-drive version is only a few pounds more than the single-drive - cheaper than converting it later. But all the extra bits will add complication for a new spinner.
If you're buying a secondhand Trad, the chances are it'll be single-drive because there are so many more of them around.