In your unit, auto mode will let you increase sensitivity to 100 percent, but the unit will limit your minimum setting. This prevents you from turning sensitivity down too low to allow automatic bottom tracking. When you change the setting with auto turned on, the unit will continue to track the bottom and make minor adjustments to the sensitivity level, with a bias toward the setting you selected.
Adjusting sensitivity in Manual Sensitivity Mode is similar to driving a car without cruise control — you have complete manual control of the car's speed. In your unit, manual mode allows you to set sensitivity at 100 percent (maximum) or zero percent (minimum.) Depending on water and bottom conditions, the bottom signal may completely disappear from the screen when you reduce sensitivity to about 45 percent or less!
Try adjusting sensitivity in both auto and manual modes to see how they work.
Bait school 
Fig. 1
Fig. 2
Fish
arches
Fig. 3
Fig. 4
These figures show results of different sensitivity levels on the same
location. Fig. 1: Sensitivity at 98 percent, determined by Auto Sensitivity. Typical of full auto mode. Fig. 2: Sensitivity set at 71 percent. Fig. 3: Sensitivity set at 47 percent. Fig. 4: Sensitivity set at 100 percent.
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