Ok this is an experimental result that has stumped me (I posted a while ago on the same topic but received no replies so this is a more specific question.) This picture shows what happens: The experiment involves dropping a mass (through a tube, visible in the top right of each shot) onto a pencil that is balanced on the edge of the table. The picture is video print-outs.. I realise the picture quality isn't great but basically, in this case the pencil jumped up backwards instead of the usual forwards (clockwise from this direction) rotation and movement. So yeah, if anyone can explain WHY or give me some clues on how to start that would be great! Thanks.