The MPPT algorithm uses the V-I curve to determine the MPP. The knee on the V-I curve represents the maximum Power Point. This point is also the boundary between stable and unstable regions. To the left of MPP, the voltage will rapidly crash to zero as current is increased; the voltage will oscillate at this point. The MPPT controller senses this instability/oscillation (following some limits) and uses it to adjust Vmp.
To find Vmp the first time, the MPPT controller increases current, starting form the right side of the V-I curve (I=0 @ Voc). Once the oscillation is detected, VMP is found.
Once Vmp is found, the MPPT controller holds the system at this voltage by adjusting the output current magnitude. MPPT algorithm perturbs the Vmp every .7 seconds. If power can be increased without the magnitude of oscillations exceeding the limit, the setpoint is changed.