Before you start play, agree a suitably heroic or action-packed genre that you will play in, whether that is high fantasy, superheroes, spy thriller or whatever you like: one player is the GM, the rest are Protagonists. The GM creates a setting with a clear goal, the Protagonists each name themselves and why they are here, but give no other details; the Protagonists then take turns to say what they are doing to advance themselves towards the goal.
The GM should create obstacles and opportunities for the Protagonists, who then respond to the situation as described: the Protagonists can do pretty much whatever is possible for those characters in that situation and should never be blocked by the GM on the grounds that their stated intent is implausible or too difficult.
In a high-fantasy adventure, the Protagonist enters a room which contains a horde of zombies; the Protagonist says "I jump towards the first one and use it to boost me up further, leaping to grab the chandelier hanging from the ceiling; I then swing off the chandelier to land at the exit, quickly ducking through and slamming the door shut behind me!" The GM cannot overrule that action, even the part where the Protagonist added a chandelier to the room.
Once an action has been declared, the Protagonist rolls a d4; after the result has been seen, the GM then announces what the target number was, which may be from 1 to 4: yes, the GM decides how difficult the task was after seeing the Protagonist's roll, so the GM has almost total power over whether any action actually succeeds or fails.
An action succeeds if the roll is greater than the difficulty number (DN), fails if the roll is less than the DN and is a mixed success if the roll equals the DN, but the GM must justify their choice of DN within the fiction. The Protagonists of the game are accomplished heroes who can achieve just about anything that is possible in that world, so it is the GM's responsibility to explain just how they succeeded when the odds were stacked against them or failed when it should have been a walk in the park.
- A DN of 1 is easy and the worst the Protagonist can get is a mixed success; when the Protagonist's roll is 3, then the threat or challenge must have been much less than they imagined or else they have a special skill which helped them to do exactly that thing. When they rolled a 4, then both of these things must be true.
- A DN of 2 is average; the Protagonist can do a bit better than average, if they rolled a 4, meaning they received help from an unexpected quarter.
- A DN of 3 is hard; a roll of 1 indicates unexpected resistance or complications.
- A DN of 4 is heroic and the best the Protagonist can get is a mixed success; a roll of 2 indicates the Protagonist has a weakness or drawback which hampered them under those exact circumstances or else the threat is much stronger than it appears to be. A roll of 1 means that both conditions apply.
- On a 4, the Protagonist fails and falls among the zombies as she just misses her grip.
- On a 3, the Protagonist makes it but drops her sword in the process, so she is now unarmed.
- On a 2, the Protagonist makes it and the task is completed as they intended.
- On a 1, the GM can either choose to have the threat be much less than the Protagonist originally perceived (perhaps the zombies are all eyeless and can't see the Protagonist) or state that the Protagonist has the exact skill or ability required to do this (the Protagonist could fill in some details about their early life as a circus acrobat.)