Why can you only rotate crank one direction?

I am doing my timing belt, and everyone says not to turn the crank counter-clockwise. Why? Is this only for interference engines?

Since you are doing the belt replacement, this will make some sense--

Turning the engine clockwise (when viewed from the front) means that the belt pulls directly on the cams, through idlers that are in a fixed position. Turning the engine the opposite way means that the first tension is on the damper. If the damper collapses much, there will be slack in the belt immediately at the crank sprocket and at the right camshaft, risking the belt jumping index. If you have the early car with the square-tooth belt, it's possible to damage the belt as it rides partially up the cogs on the sprocket. On all cars you'll risk having the cam out of time.

So with a properly tensioned later-style belt installed with a damper that's in perfect shape, you probably won't have a problem. But why risk it?

That's my analysis, anyway.

'89 S4 Auto, black