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books/core/54.html

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+<h2>A THRESHOLD FOR FAILURE</h2>
+<p>
+  If you use a Clock to represent a complex series of operations to achieve a
+  goal, you must also establish when that goal will be irredeemably lost: does
+  it take a single failed Check, or is there a parallel "failure" Clock that
+  fills whenever a character rolls poorly? The Game Master determines this on a
+  case-by-case basis, but must make sure to inform the Players as they approach
+  the challenge.
+</p>
+<h2>TURNING BACK A CLOCK</h2>
+<p>
+  Characters can also take action to slow a Clock's advance and erase some of
+  the filled sections, eventually bringing it back to 0 — the method is
+  identical to that of filling a Clock. When a Clock reaches 0, the Game Master
+  may have the threat removed or simply keep it lurking in the shadows.
+</p>
+<p class="example">
+  Example: As the heroes fight the powerful sorceress Chrona, her ritual to open
+  a rift to the Demon Realm draws to its conclusion. The Player Characters must
+  balance their actions between defeating the sorceress and slowing down the
+  progress of the ritual, a ten-sections Clock that would end the scene in an
+  automatic defeat.
+</p>