I have been using Revit for over 5 years now, and I don't think I have ever used the Show Hidden Lines tool. Am I the only one? Basically, its a two pick tool, first pick for the 'front' element, second pick for the 'back' one - then, then lines from the back element will show through as Hidden Lines.
This tool is hiding on the View ribbon panel:
From the Wiki: After you put detail components in the correct graphic draw order, you can use the Show Hidden Lines tool to obscure the lines behind a detail component.
via Show Hidden Lines - WikiHelp