Just worked it out…You need to hold down option, then drag around one of the circles which appears (under your cursor, and in the opposite corner relate to the screen’s centre). The other moves in the opposite direction relative to the centre of the touchpad. If you hold down option and shift, you can move both circles together in the same direction (though I’m not sure what that’s for).What’s surprising is that I only now realise that you can’t pinch into a specific part of the screen (even on a real iPod touch), it always zooms into the centre.