When swearing at somebody behind their back,
Then when they turn around to look at you, pretending to do something else with your hands; such as scratching your face.

The very first person to do a schafernaker, was Thomas Schafernaker; BBC weatherman.

Whilst making rude gestures to the news reporter, camera flicks to him live, as soon as he realises he is on air, pulls an amazing schafernaker!!