That is tied to the measured resonance frequency of the blade (pretty close anyway)
When You look at hipnotic's post Freq.: 1320Hz
approximate frequencies:
- 1,000 DEF
1000 - 1,100 ALL
1,100 - 1,250 ALL+
1,250 - 1,400 OFF-
1,400 - 1,600 OFF
1,600 - OFF+
This puts the blade in an OFF- range...