There are loads of variations of it....you can also get creative and use it with your bag workouts.
I just got a stopwatch that has a setting on there for the timing. If you wanna make your own...here's what I did:
Buy a cheap wall clock from Argos (Approx. £4). Take it apart and remove the hour hand and minute hand (leaving only the second hand).
While it's still apart mark the face with a neon pen. (see attached file)
Put it all back together and your ready to go.
You can also play with the timing ratio's, i.e. 1/1 (10 secs on/10 secs off) or 2/2 (20 secs on/20 secs off)
there's loads of info online, but the best one I've found is
HERE
Hope this helps.
Ash