Tension pool mechanics are a simple system I've used in several games, now. I find they work best in duel situations: while they could theoretically rebalance to allow for three or more players in the situation, I have not had to do so. Instead, I find that duels proceed so quickly (and entertainingly) that the sideline players are not bored, and the headlining player is encouraged by their attention to be ever more hilarious and interesting. Plus, if it's PvP, the players can fight each other without needing a big team or a GM.
The idea of a tension pool is that you can't really win a fight right off the bat. In a normal game of, say, D&D, you're doing damage right from the first round. To keep the combats long enough to be entertaining, this requires that everything have eight billion HP. It doesn't make any sense - and isn't very dramatic - that you have to hit a wolf four times with a sword in order to kill it, and it never really reacts to the damage in any kind of meaningful way.
Instead, an encounter with a wolf using tension pool mechanics would probably start with the wolf leaping for your throat. There would be some back-and-forth: the wolf springs around, you land glancing blows, but he gets inside your arc and is biting at your neck...
But all of this isn't done by rolling a d20, checking his AC, then looking to see how much damage your sword does. Instead, these moves are played and narrated, and tension is added (or, rarely, removed) from the pool. As more tension is added to the pool, moves have more effect.
If the wolf comes inside your reach right at the beginning, the turn would end with you battering him away, maybe with a few claw scratches. However, if you've been fighting for a while and the tension pool is high when the wolf comes in, that wolf might bite your wrist, disarming and wounding you. Such an action would reduce the tension pool.
To look at it from another example: two Jedi fight. The tension pool gets high. One Jedi uses a disarm card. Early in the fight, it would be a spinny, splashy piece of swordfighting. However, there's enough tension in there to spend it and actually disarm him: his saber goes spinning away. (Cutting off his hand is usually considered ending the duel, which is a bit more expensive.)
Now, however, the tension pool is much reduced! This means that the attacker cannot simply stab you and kill you, because there isn't enough tension. Instead, there will be desperate, flashy dodging and hand to hand as our disarmed hero tries to get enough breathing room to pull the saber back to his grip.
Obviously, bringing fingernails to a light saber fight is a losing proposition, and it is a rather desperate situation. But the point is that he is not simply slain. He has time to react, time to be interesting, maybe even time to get rescued or turn dark-side.
Even if he is outmaneuvered in terms of combat, he may still play cards that give him noncombat effects. He can, as mentioned, try to turn to the dark side. If he's clever, he'll play that blind guy walking through at this point. There are other options and resources he might think about using... it can be as deep or shallow as you please.
Tension pool can be used either with multi-purpose cards (does THIS and, at high tension, THIS, and THAT, if you can afford it) or it can be used with stepped cards (this can only be done at five or more tension).
It's possible to use multiple tension pools (one for each player, say), but I find this to be a bit pointless unless you're really looking to extend individual duels to more than twenty minutes of real time. For the vast majority of games, duels shouldn't take more than five minutes each.
Of course, tension pools are all about creating a level of drama rather than any kind of realism. Therefore, tension pools don't have to be about combat at all.
Imagine that we use the same rules, but apply them to a narrative. Instead of the GM coming up with all the narrative, the players and the GM "duel" over bits of the narrative using cards.
Here's a thought example:
The players are all from Assward Village, population 412. An evil wizard named Jim-Bob has set up shop near by. Jim-Bob the Thaumaturge has begun to poison the land and capture some of the villagers, so the players set out to take him down.
They play an "approach" card - they walk on over to Jim-Bob's Wizard's
The GM plays a "monsters!" card. Because there's not much tension here, the monsters aren't really all that cool. A bunch of zombie ferns attack the players. If the game has combat details, there can be combat. Otherwise, you narrate the player's victory. Two tokens are added to the tension pool.
The players agree to play a "scale the walls!" card, which lets them get over various obstacles. There are no obstacles for them to get over. They played it just because it allows them to move forward a story phase by spending three tension.
So they scale the wall, climbing into Jim-Bob's Vertical Mobile Home. Phase two begins, the GM switches decks...
How complex this could get is arbitrary. While Jim-Bob is a rather simple threat, it is also quite possible to make the end boss ridiculously overpowered, and allow players to deal with lesser threats first, each of which reduces the end boss' power a bit. There are a lot of other, more story-fiat options.
Also, a good option would be to have a series of tiers: the gross tier ("approach the boss", "monsters attack") and one or more finer tiers (the actual fight, the journey through a now-dangerous forest). Every large-scale action involves dueling on smaller scales in order to actually get it done... and maybe build up some resources for later use.
The available options are limited only by cards and rules. How interesting it will be to play is largely a matter of how good you are at creating interesting card games.
A big factor here is longevity.
In most games, longevity is achieved through gaining levels, equipment, and so forth. In tension pool rules on combat, the same method can be used. However, with tension pool rules on story, you have to be a bit more clever.
The same basic mechanics will serve: the player needs to be able to change his deck, get new cards, and occasionally even gain a fundamental new power. This will give him enough to focus on so that he can have fun with the real fun part of the game: interacting with other players and the GM from various angles.
The iffy part is how to link that up emotionally, in the player's mind.
One way to do it is to make the player only partly his player character, and also partly something else. A concept. A god. A family. An ancestral spirit. A government. The sky is the limit, but you need to be careful to make sure it is emotionally involving.
Do you see what I'm talking about?