Say you want to move students into groups. Setting a 30-second limit and playing some suitably frantic music helps cut the casual chat and zombie pace. Things are further improved when the movement itself has a purpose. For example, I might give them the 30 seconds to find someone who can answer a particular question or complete the other half of a sentence.
Timers are also an easy way of energising starter and plenary activities.
Putting a problem on the board at the beginning of the lesson and giving an appropriate amount of time to answer it gives the students a focused start to the lesson. In English, for example, editing a short unpunctuated paragraph becomes more fun when it's a race against the clock. And a plenary, in which pairs talk about or write down whatever they've learnt that lesson, is made sharper when there's a clear deadline.
However, there are times when you wouldn't want to use a countdown.
Projecting a giant red clock and playing Motorhead's The Ace of Spades during a spelling test, for instance, would probably not be helpful.
Generally, though, an onscreen timer is a simple way of using ICT to liven up any number of classroom activities.
Jon Eaton. English teacher, Kingsbridge Community College, Devon