To set the timer place it down with the desired time (5, 15, 30, or 60 minutes) facing up. When your time's up, a loud alarm goes off. To silence it, turn it so the 0 is face up.
I used it 10 times in a row when I was roasting pork ribs last week (I needed to flip them over every 15 minutes). I also use it when I'm working if I want to give myself 15 minutes to go over my purchases on Mint.com or some other task I abhor. Some people use them to train their kids.
If you want a different set of times, Datexx offers four other cubes: Blue – 0,1,2,3,5. Green – 0,1,5,10,15. Yellow – 0,5,10,20,45. Purple – 0,5,10,20,30.