Just as the name states, it's a shaft that is in between the crankshaft and the camshaft in the car (connected by chains to each). It is located below the crankshaft (and the flywheel is attached to the end of the crankshaft) and sits between the 2 halves of the engine case. A chain failing in there would be catastrophic to the engine, because all of those moving parts would collide and obliterate each other. But the more common problem is the intermediate shaft seals at each end of the case can develop a leak, much like the rear main seal at the crankshaft. I just had this repair done to my Box last week, which is why I am fresh on what the intermediate shaft is....
