When the early Salesians in Argentina were finding it difficult to apply the Preventive System, Don Bosco wrote a letter to don Costamagna who shared it with the others. The subsequent spiritual and temporal prosperity of the Argentinian province was attributed to this letter. What strikes me is that some salesians who were finding it especially hard to be patient and charitable obliged themselves by vow to do so, and they used to renew this vow every month, at the exercise for a happy death. A fourth Salesian vow while Don Bosco was still alive!
Ceria: Don Bosco's system produces good pupils because in the first place it produces good educators.
Caviglia: the Preventive System is Don Bosco.
Albera: the Preventive System is the magna carta of our congregation.
(See The Salesian Rector, nn. 109-110)