Seth Godin wrote:

Thea von Harbou invented the countdown. 10, 9, 8…

It works.

It focuses the attention of everyone involved and ensures that we’re truly alert for what’s going to happen next.

It helps that the numbers go down, not up (because up might never end). And it helps that as we get closer to lift-off, tension goes up, not down.

Seth's post reminded me of a video I made years ago about how I used a countdown timer to more than double clicks on an ad on my website. So yeah, I can verify that countdowns work.