Rule 7 of 24 · Chapter II — Systems and Their Price
Draw the edges of the possible
Why this rule exists
A reader can only feel suspense about a power whose limits they roughly understand, because tension is the gap between what a character needs and what the rules will let them do. Soft, mysterious magic can be gorgeous and is wonderful for wonder, but the more a magic is going to solve problems, the more clearly the reader must know its boundaries, or every rescue reads as a cheat. The distinction is not hard versus soft as a matter of taste; it is a promise about how the power may be used to win. Define the edges of what a system can do and the reader can play along, anticipating, dreading, hoping, instead of waiting passively to see what the author will permit.
In practice
Decide, at least for yourself, how your magic or technology works and where it stops, then reveal as much of that to the reader as the story will lean on. If a power is going to resolve conflicts, show its rules and limits in low-stakes scenes first, so that when it matters the reader already knows what it can and cannot do. Reserve the deliberately unexplained for wonder and dread, the things that awe rather than solve, and keep your problem-solving magic on the well-lit side of the line. When a solution lands flat, check whether you asked the power to do something the reader had no way to see coming.
When it doesn't apply
Mystery has its own power, and a story built on awe rather than problem-solving may keep its magic almost wholly unexplained, as myth and fairy tale do. The danger is only when unexplained magic wins the day; wonder that is never asked to save anyone owes the reader no rulebook. Match your clarity to the job the magic must perform.