By taking out all the harmonics in a square wave that are multiples of three it creates a “stepped square wave” that resembles a blocky sine wave. In fact, if there was a sample and hold type setup with the sample rate 6 times the frequency of the sine wave it’s sampling, then it would actually produce exactly the same waveform as this, only with sharper edges.
Edit: Sample and hold setup as described in the description has been added.
