The idea of the Tension Pool is that we are missing ways to:
- Illustrate the idea of time passing
- Display a level of danger in the area
- Connect player character actions and relevant consequences
- Give the players a tangible, visible sense of the above, which would mirror the character's feelings of impatience or dread
A great many thanks to the Angry GM for coming up with the idea, the original concept is linked below. Angry goes into far more detail in how it would be implemented and the psychology behind it, so this is a simple summary for reference. The rules listed here will likely be a little bit different as they are iterated and refined and adjusted to meet particular needs.
On the table in a very visible spot is the Time Bowl. When in a situation that is not totally safe (dungeon crawling, wilderness travel, city wandering, whatever), dice are added to the bowl, each marking a duration of time.
When enough are added to the pool, all the dice are rolled and if there's a 1 showing, that's a trigger for a random event, usually an undesirable one. If this happens, the Time Bowl is emptied and everything restarts.
If the pool is filled and rolled and there aren't any 1's, keep adding dice and re-rolling until there is.
If the party does something loud or stupid or would otherwise cause a ruckus, roll the dice pool as is. Resolve a 1 if you roll it, but don't empty the Bowl this time. If there is a 1, them resolve the event and remove one die from the Bowl. This balances the frequency of rolls a little.
The type of die that is put in the pool depends on the level of danger, or the odds you want a random event to occur. The smaller the die, the more likely they will trigger. This could be the alert level of a dungeon or city, how loud the party is being, or simply odds based on how many events you want to have happen. A failed stealth roll might make the time dice smaller.
The duration tracked by the Time Bowl depends on the situation. In a dungeon, each reset of the bowl may be an hour. In the wilderness, each reset roll may signify 8 hours or a day. So depending on the setting, adding a die to the pool can represent 10 minutes passing, or 1 or 3 hours passing. In a sci-fi setting, a pool reset may mean a week in space, with each die signifying a day.
No matter the setting, the passing of time in any RPG is a very fuzzy thing (or else GMs wouldn't resort to saying "time passes.."), so it's not critical that the number of dice in the time pool equate exactly. As long as a reset means about an hour passed, and a die added means a few minutes passed, no one should be asking any questions.