1979-1981: historical question

[Eric Albert]

Just now, while reading the book _Basketball on Paper_, I learned that between 1979 and 1981 a team's coach was ejected if their team had three illegal defense technicals called against them.

I started watching the Celtics near the end of the 1987 season (yes, I believe I must be close to the record for "C's fan who's watched the most games without seeing them win a championship"), so I missed those years. But I'm fascinated. How often did this happen? What constituted illegal defense back then?

-- Eric