Create a 3D Cutaway on the Floor Plan or Section

To cancel the operation: Note that the tabs on all four sides change into garbage cans: if you change your mind while dragging, just move the Cutting Plane back into one of the garbage cans (or press Esc) to cancel the operation.
Select the Section line, and choose Create Cutting Plane Here.
Choose Create Custom Cutting Plane.
Note: This command is also available from the Standard Toolbar.