I wrote a short english manual for the flash version here.

- The frame / rings / disks / cursors can be dragged by the mouse.
- The center point is the "precise mode" on/off switcher.
- The red switch change sides.
- Click on the 1h mark ("calculating" side) to lock the cursor.

If the webpage border is too wide / monitor too small, use the clean, borderless version: link icon
If you want to download the .jpgs and build one, just click here: link icon
I recommend to read the Roland van Riet's document, the Dreiekrechner 2009.pdf, which downloadable from here (english): link icon