Shake to start, or just be patient.
When the voltage is just above 0V it can activate the right transistor and draw current, the amount increasing with voltage as expected. Right around 6V the left transistor starts to activate and draws away the small amount of base current available to the right transistor thereby shutting it off and decreasing the current to a much lower value. Thus right at 6V the current actually decreases with increasing voltage and increases with decreasing voltage. This is negative dynamic resistance and can be used to create a negative damping factor in an LC circuit thereby causing it to oscillate. The voltage is bounded by the fact that only a small region of voltage has negative dynamic resistance and once the oscillations go outside that region it encounters positive dynamic resistance, and once the oscillation is large enough to spend most of its time in the positive dynamic resistance zone it stops growing.
