How does the shot clock work in pool?

Question

I was watching a little bit of a pool match. It appears that a 20-second shot clock was used, as they would sometimes display a countdown starting at 20.

But it's not clear to me how the shot clock worked. For example, at 45:35, the player takes a shot and makes it, and then the cue ball comes to a stop at 45:43. But then the shot clock doesn't start until 45:53. Then at 46:04, the shot clock is down to 9 seconds and it disappears, but the player hasn't taken his next shot yet. Then at 46:23, the shot clock reappears starting at 20, and the player takes his shot with 9 seconds left.

So my questions are:

• When does the shot clock start?
• What causes the shot clock to stop and reset before a shot is taken?

Answer 1

It appears different organizations/tournaments use different clocks.

This one looks to me like it's using a 30 second clock (starting after all the balls from the previous shot come to rest). I think the TV graphics just don't bother showing the first 10 seconds of the count, but you can see the countdown "ring" is already about a third gone when the "20" appears.

In some tournaments, each player gets one time extension per rack. WPA seems to prefer 25-second extensions. To me it looks like the player has called for such an extension while considering the shot at 46:04. When the clock reappears, it's about 30 seconds behind where it was previously, so this match might have used 30-seconds instead.