RailwayRouteOptimization/scripts/Modify/BreakOutManual/doc/BreakOutManual_proc_en.html

<ol>
  <li>Select the entity you want to break up.</li>
  <li>Click the first point at which the entity is to be divided.</li>
  <li>If the point does not lie on the entity, the separating point is assumed
  to be the closest point on the entity.</li>
  <li>Click the second point at which the entity is to be divided.</li>
  <li>The segment between the two points is removed if the "Remove Segment"
  option in the options toolbar is active. Otherwise, the entity is only
  separated at the given points.</li>
</ol>