Start the real tone or its digitized version and sweep the frequency slider past half the sample rate (4 kHz by default, so Nyquist is 2 kHz): the input keeps rising while the played-back pitch folds and descends. The plot shows the samples, the input wave and the dashed alias that passes through the same points; the digitized tone is synthesized at the folded frequency.
The claim above is what this widget demonstrates. Disagree with it?