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Activate the checkbox and you will see a viewport gizmo in the form of two yellow boxes. The space between the boxes determines the falloff area.With different “falloff options” the gizmo will change accordingly to a sphere or plane, but the mode of operation is still the same.
In order to transform the falloff area change "Falloff Bounds"/"Falloff Objects" or use the manipulator tool from the "Line Falloff" section.
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