What is a Microfront?
Microfronts are a type of custom front for prepping interstitial scenes during a campaign. Ultimately, microfronts are a basic front that has been hammered down to its most compact state. Microfronts act as interludes in a longer campaign by providing a short diversion between story arcs. While they could be extended to form a whole front, they are designed to be resolved in a single scene.
Writing your own Microfronts...
Microfronts are just like a regular front. They have all the same parts: A description, a danger, grim portents, an impending doom, and perhaps a custom move, item, or monster. The trick is to write them with such immediacy that they resolve in a single scene. This means that the description needs to drive toward direct threat, the grim portents need to be all but givens, and the impending doom needs to directly impact the players right here and very much right now! Could you expand a microfront into full front if things get interesting? Sure, you're almost obligated to. Writing a full front just isn’t what you're setting out to do.
To really ratchet up the tension...
...put microfronts in envelopes and write their fictional triggers on the outside. Hand them to the players at the beginning of the session, and let them decide if and when they want to activate a microfront.