Imagine the 15Mohms resistor as a gad pedal, whenever you push the gas pedal resistance decreasing, and it will open the way to the left-transistor to allow current to flow to the right-transistors, allowing the 5V battery circuit to complete and finally the RELAY will shift the switch automatically, allowing the RPM to activate.
[Decrease the 15M ohms resistance and let electrons to find their way]
-This circuit made my professor @ahmedzz that's his everycircuit account -
