During the Montreal Canadiens vs New York Rangers game on April 16 2017, the referees called an illegal substitution on the Rangers, but strangely not too many men on ice. This happened during the second period as some players were changing shifts. How come this didn't result in a 2 minute minor? This is a reference to this incident, I wasn't able to find any other similar occurrence.
so I'm wondering what is illegal substitution and when does it happen? How does it correspond to the too many men on ice penalty?