If I try to use a plectrum like in the pictures it just rotates anticlockwise.

If you hold a plectrum as in the picture but fold the second finger behind the thumb so its tip is to the right of the first finger tip (preventing the rotation) that is fairly close.

You can't say 'this is how it is done' - despite me having a ferocious grip in both hands I can't maintain enough pressure to keep a plectrum from rotating in a two point grip.

I used to know someone who constantly lost plectrums (plectrii?) into his guitar. He used to get some of them out by shaking it but whenever he changed the strings I was asked to fish out the stubborn ones - I was the only one with hands/ forearms small enough to reach right inside. The first time I did so I found about a dozen, some of them he'd not seen for years. Just what made his method of strumming likely to propel the plectrum into the sound hole I do not know, but he would lose two or three a night.

Anne