Apropos 2.0 jtd.
My book doesn't mention it but just seen another peugeot manual that says"IMPERATIVE:Never turn crankshaft in reverse direction".
What happens if you do?

Well apart from drawing air in through the exhaust valves, and out through the intake....

apparently it's possible with old diesels to run the engine in reverse if bump started backwards lol. no idea as to why you shouldn't turn it the wrong way on a modern diesel though?

Weaken or shorten the life of the timing belt due to being stretched the opposite way. Valves could contact piston crowns because of slack in timing belt. Timing can slip a tooth. 3 good reasons

the tensioner is positioned in a way that means it can only maintain correct tension in one direction. you can turn an engine backwards slowly by hand using a ratchet on the crank pulley, but you must be very careful not to cause valve/piston contact because the belt will sag between crank and cam and could slip.