I found myself trying to dig down into a stack of documents on the corkboard using the ‘unpinch’ (I don’t know how it’s officially called) gesture on my trackpad.
Wouldn’t that more intuitively be used to change the scale of the corkboard cards, i.e. “zoom” in and out? As I recall there were technical issues with implementing that idea. But if you want better control over shortcuts, gestures and so forth, I definitely recommend looking into a customisation tool like BetterTouchTool or Keyboard Maestro. I can very easily add this gesture (and hundreds more) to the corkboard, if I wish to.
At any rate, without customisation, you can just double-click on the card icon to drill down—or more easily in my opinion, simply hit the spacebar with it selected.