Piston propeller check dilemma - which way?
Hello there once again,
I have been listening to contradictory information regarding how to check for a piston's compression in a pre-flight.
When doing your pre-flight inspection, you turn the propeller (magnetos off) with your hands as many times as the number of cylinders in the engine. I've always heard this was both for a little lubricating before starting the engine, and also to check for the piston compression. If it's not a little hard to turn the propeller after a small amount of movement, there's a problem with the compression of one (or more) of the pistons.
Now (and critics and feeback welcome about the above procedure), I heard two versions about how you do it.
First, I heard you always rotate the propeller in the OPPOSITE direction of the normal propeller movement (with engine running). This is for safety since the magnetos would not "springload" and generate a spark, and maybe the engine could start if there's some avgas in the combustion chamber. And I was ok with this, since you only hear the magneto "clicking" when you rotate it in the same movement as with the engine running.
Now recently I heard that turning the propeller in a contraty movement of the normal one is completely wrong, because the vacuum pump could damage if you do that.
Now, I'm confused. Both ways allow you to check for the pistons compression, and rotating it in the opposite direction helps you not to harm yourself even in the case the magnetos switch are off but there is the rare case of faulty wiring or bad contacts that are not really turning the mags off.
I want to know what you think about all this, because I also never read a POH/AFM that listed rotating a propeller manually to check anything, compression, or anything like that.
And if its really a good procedure in a pre-flight, how would you do it? Normal movement or opposite movement, and why?
Thank you in advance.