Though the data doesn't directly illuminate how effective principals get these gains, it does shed light on the relationship between teacher turnover and effective school leadership. Teacher turnover was highest in schools with least effective principals, independent of student poverty rates. In schools with top principals, there was a negative relationship between teacher turnover and the average value-added scores for the grade. In other words, the better students performed, the less churn there was in staffing.
The paper seems to confirm what common sense suggests: once a good principal gets the right teachers in the building, he does what he can to keep them.