Final Countdown

Five.
Four.
Three.
Two.
One.

It's the final countdown! [cue music]

This particular trope tends to make its home in thrillers.
From the bomb in the London Underground in BBC's Sherlock, to the faulty timer in Galaxy Quest, the ticking time-bomb is an easy trope. It's a simple way to build tension quite quickly. Sometimes, it's subtle and sans obvious tension and stakes, like in "Bad News" from How I Met Your Mother. The episode slowly counts down from 50 until there's nothing left but the realization that Marshall's world will never be the same.

Popular cult-classic dramedy "Chuck" had its titular character disarm many a primed-to-blow bomb. Sometimes it was with a computer virus sent from the site of a famous adult actress, or with something as simple and asinine as fruit juice.
When it's done well, it's done very well. We the audience can only hold our breaths, bite our nails, and try to keep our butts on the edges of our seats. We wince when the characters mess up and when we get to that final 5-4-3-2... We gasp. We laugh. We let out a sigh of relief. Our heroes managed to save the day yet again!

As you may have deduced by my references this time around, this particular trope works best in more visual medias. I've yet to find a short story or novel that captures the potential paradigm shift of the countdown.

Sometimes, though, it's done cheaply. Time seems to skip and freeze as our beloved directors see fit. Twenty seconds left on the clock? Pfft. More like 120. 5 minutes left? Wait, no... Hold on; the hero spent too much time staring longingly at his sidekick and now we've only got 54 seconds left!
If you're going to use a timer, please, please obey the laws of physics!¹ Furthermore, don't forget that we as the audience need to care about the stakes. If you stick a countdown in a story filled with horrible characters and a lackluster plot line, it won't matter. You can even stick four, three, or two countdowns² in there and it won't change a thing. Use the countdown well and you can strengthen your plot and the audiences' bond to the characters. Use it badly, and it won't change a thing. Be aware of how to use a countdown, and do it well. Your audience will thank you for it.

I feel the need to add a side note. Timebombs don't always have timers... Sometimes it's nothing more than a catchy beat...

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1 Unless it's Doctor Who, in which case you are free to do as you wish. Lord knows Steven Moffat does all the time. Also; if you could introduce me to Matt Smith, I'd be ever so thankful^↩
2 See what I did there?^↩

For more information, feel free to consult http://tvtropes.org/pmwiki/pmwiki.php/Main/TimeBomb ^↩