Safety valve mechanism is:
There is little change in the width of the dental arch anterior to the first permanent molars after the fifth or sixth year of life. 
In the mandibular dentition, mandibular intercanine width is relatively complete by 9–10 years of age in both boys and girls.
In the maxilla, intercanine width is essentially completed by 12 years of age in girls, but continues to grow until 18 years of age in boys. 
Hence, the maxillary intercanine dimension serves as a ‘safety valve’ for this basal discrepancy.
The anteroposterior increment in the maxilla at 14 years
Increase in the mandibular intercanine width at 14 years
Increase in the mandibular height at 12 years
Increase in the maxillary intercanine width at 12 years