When speaking with PMPs, it surprises me how many are unaware of what the “F&S” stamped on the yellow expanded trigger of many snap trap brands stands for. It does not stand for “fast and slow,” but rather “firm and sensitive.” The expanded trigger allows you to adjust the sensitivity based on where you place the locking bar when setting the trap.
When setting on the “F” side, it will take more effort to fire the trap. The firm setting would be used in areas where vibration may be an issue, such as a warehouse with tow motor activity. When placed on the “S” setting, the trap will fire with the slightest touch or vibration, which is likely too sensitive.
Most times, for best results, I suggest setting the trap in the middle. In addition to raising your success rate, this also may prevent you from getting your fingers snapped!
from Pest Management Professional http://www.mypmp.net/2018/09/02/setting-traps-with-an-expanded-trigger/