Further quirk in the drag+reference is that when usingCmd+Opt+Drag, ensuring the Cmd+Opt are depressed before the drag starts (otherwise different data is loaded). This is a constraint in the design of Bookends so you just have to act as described.
More on [Bookends & Tinderbox](http://Use a Cmd+Opt+Drag, ensuring the Cmd+Opt are depressed before the drag starts (otherwise different data is loaded).