When your character wants to do something interesting and risky that affects the gameworld, what you’re looking for is a move. There are basic moves available to everyone (fight, investigate), and there are special moves that are limited to specific playbooks. When you decide to do something listed as a move, (usually) that’s when you roll dice.

The rule for moves is to do it, do it. That is straight out lifted from Apocalypse World, and it means that when you want to make a move, you have to do the thing listed in the move. i.e. “when you attack a vulnerable enemy in melee combat…”, then you roll dice. It works reciprocally, too. If you attack an enemy in melee combat, expect to roll dice and use the rules for Fight.

At any point the MC should and will be asking you “and how do you do that” and “and now what do you do”

The game mechanics part of making a move is (generally) rolling two six sided dice, adding them together; and adding an additional modifier usually based on stat. For instance, Fight says “roll +STR”. You roll 2d6, and add your Strength stat to get a total. PBtA games in general and Mutant World in specific doesn’t slap a difficulty penalty on top of that, but does use a general category of “Advantage” (+1) or “Disadvantage” (-1) for specific conditions that affect the roll.

And then you compare your total to this totally easy to remember result table:

• On a ten or higher, you get a strong success. You get the good stuff without the bad stuff.
• On a seven through nine, you get a weak success. You get the good stuff, but be prepared to pay a price.
• On a six or less, you get trouble. You get bad stuff, hopefully interesting bad stuff. You may also get some good stuff, depending on how risky what you were trying is. (Powered by the Apocalypse games usually call this making a soft MC move or making a hard MC move).

All of these outcomes are influences by the player and the MC; usually what strong success looks like is most like what the player is trying for, and what trouble looks like is what the MC is cackling about in the corner. Usually.