I read about this from UK gardener Sarah Raven.  Planting fava beans alongside potatoes will give the potatoes a nitrogen boost, and by the time you need to hill them, the fava beans will be harvested.  So far I have hilled the potatoes but I'm still waiting for my fava beans to appear.  They are starting now and when they're gone, I'll hill the potatoes once again.  Maybe this crop will prove to be enormous!