How did Joel Greenblatt achieve 50%/year returns between 1985 and 1994?

Joel Greenblatt has one of the best investing records of all time.
Between 1985 and 1994 he achieved annualized returns of 50%
($7 million into $500 million in 10 years)
In 1995 he returned all capital to the outside partners and continued to invest his personal funds.

This recording is from Apr 9, 2015.

Joel Greenblatt teaches a value investing class at Columbia Business School.
You can watch these lectures for free on my youtube channel: