A countdown takes place when there is an unknown time limit before some event happens.
A typical usage of this mechanic might be to determine when a character’s death occurs. Say a character has just suffered a Near Death injury by any means: they are now dying. But when do they die? A countdown can determine when the event happens.
A countdown is a Situational Roll. For a regular slow countdown, a “tick” is any result that is +1 or above on the dice roll. Remember that in Situational Rolls there are no modifications to the dice results. The tick threshold can be raised or lowered depending on how fast or slow the GM wishes the countdown to run, but +1 or above is about right for a regular countdown.
The next step is to determine how many of these ticks it takes before time runs out and the countdown ends. For our Near Death character, I have a Stamina Attribute and use it as a guideline by assigning a numerical value to each level of that Attribute. For example Fair is set at 4 ticks, and it goes up and down from there. The higher the Attribute score, the higher the numerical value.
If the character has reached Near Death from combat then I would say the countdown should be in rounds. Each round a Situational Roll is made, and anytime it comes up +1 or higher, it counts against those 4 ticks. If the character is rescued or otherwise stabilized and no longer Near Death then the countdown stops. If on the other hand, all 4 ticks get used before help arrives, the character dies.
How often the rolls are made can be determined by the circumstances, just as the target number is determined by circumstances. Besides combat, countdowns can be used for many things: how long a character can hold their breath, when a damaged spaceship explodes, or illnesses or poisons that impact the character as the symptoms or effects worsen. Any number of uses can be created for a ticking clock dice mechanic.