Report Layout - new page per...

Anyone know how to force a new page after a group has been printed but make it optional?

I can always force this be selecting 'Edit Group' then 'page break after' but inlike most options this is a straight checkbox and you cannot enter an expression to determine whether you should do a pagebreak depending on a paramater. I have loooked at a standard report that does this and although I undertsand how top pass this as a parameter to the report I can't see how this is done on the report itself.