good practice for using a circular saw
Holding the saw with one hand is an advanced, and unsafe technique. You'll often find that folks in the construction industry (especially those on YouTube), do not follow proper safety precautions. 

Holding the saw with two hands will reduce the chances of the saw kicking uncontrollably, if the saw happens to bind or hit a foreign object.  It also insures that both hands are in a safe place while cutting.