You wouldn't pet a cat backwards would you?
There is quite a bit of extra chain, especially the one from the crank to the IMS. The slack is always on the side of the tensioner, while the non slack side keeps perfect (cough) unison. Turning it backwards will pull the slack side tight, and allowing the "extra" chain to now destroy the relationship of the two gears.
If the tensioners were rigid it wouldnt matter much, but they are a combination of a not particularly ballsy spring and a piston trapping oil behind a check valve (I'm simplifying for purpose of the description) These tensioner are already a source of trouble (look up chain rattle on start up)
This pic shows the chain I'm referring to. The zip tie is near the point where the tensioner pushes
Even if turning it backwards wasn't enough to cause valves to touch pistons it would make it very hard to get consistent timing. You would have to spin it the correct direction several times in order to get it to settle in.
Once that zip tie was cut i put sharpie arrows on both ends of the engine to keep me from doing something dumber than usual.