The MIDI stop value is the defualt MIDI value sent to the computer when you turn the iMoov off or switch to a new controller. Alternatively you can turn this off to allow the MIDI value to hold on the last MIDI note sent. We have provided with some common options - Min, Mid and Max.
For example, if the iMoov was controlling a huge reverb, you could set the default MIDI value to min and, during performance, generate a large soundscape by moving the iPhone to the maximum point on the movement to emphasise a build up and the hit the orange button on the beat to snap the effect off for the drop. More example usage like this can be found in Effect Mappings.