Dude "Con Rods in the Vacumn pump as described above!
If you compress a fuel air mix and get a spark, the direction it will initially go will depend on where the piston is in relation to TDC; it will always go down the bore and could be in either direction depending which side of TDC it is. I have seen an engine kick back so I would never dream of putting my hands on the wrong side of the prop.