Why turn it backwards? The engine is designed to run forwards and timed accordingly. If you turn it forwards and it were to fire at least you know which way its going to go and arrange your hands accordingly. If you turn it backwards it could fire either way and chop your hands off. If you are checking the compression as required in the A Check CAP411 LAMS Schedule you need to turn it the correct way.
The Vacumn pump is designed to operate in one direction only, if you turn it backwards you run the risk of damaging the connecting rods which can snap off if they are pushing when they should be pulling.
DO NOT TURN AN ENGINE BACKWARDS!