There have been discussions about turning engines via crank bolts before on this BB and I thought I would recap some of the logic behind this policy to make sure people know it is fact, not fiction.
The crank gear is the drive gear, and the cam gear is the driven gear. A chain is strictly a tension load carrier. You can not "push" a chain or rope, only pull it. The drive chain design is such that the driving side of the chain is in tension, as it should be. The chain tensioner, or belt tensioner on japanese cars, is on the unlaoded or idle side. When you attempt to rotate in the wrong direction, it puts the idle side in tension, causing the tensioning device to spring back, and then the chain or belt jumps a tooth. The rest is history. Rotating in the correct direction, it will go hundreds of thousands of miles whether driven by the pistons or your rachet wrench.
Hope this helps!