It's simply a standard pinpoint axle with a waisted brass bearing soldered to each end, then a couple of diagonal slits cut in the side of each bearing with a slitting disc in a mini-drill or a razor saw.
To use it, spring the vehicle's w-irons apart to extract the OO axle and insert the tool in their place. Twiddle the tool for a while until it cuts the required countersink. Extract the tool, insert bearings in the now countersunk holes and insert your P4 axle. The bearings should automatically be the correct distance apart
My first vehicle using this tool has come out perfectly square, it will be interesting to see how it runs compared to an identical vehicle that I've fitted with springing instead.
Hope this is of interest, and please excuse the quality of the photos,
Simon