The belt threads through the "roller" on the closed side of the buckle. The space for the belt is narrow. If your belt is new and stiff, it may take a bit of pushing to get it through the slot. (That's what I found out the hard way). But this is a great buckle otherwise--no slippage! It won't come open!
CHRISTOPHER D Mary,
Thanks for your answer, your answer is the best so far but I'm sorry I still can't figure it out. If you can explain it better or/and show me pictures, I would appreciate it.