One die fits onto the long handle, the other onto the watch.
There are six dies and the sizes are:
18.5mm, 20.2mm, 22.5mm, 26.5mm, 28.3mm, 29.5mm, 29.5mm
Now I know this is remarkably cheap for a Rolex opener, do remember that you get what you pay for:
a) maximum size is 29.5mm so it won't open very large watches
b) you probably won't be able to hold the watch in one hand and operate the opener in the other, you will need a case holder and vice.