When the voltage (charge) of the gate is changed, more or less electrons are attracted from the p-type substrate area. Although there are a lot of holes in the p-substrate there are still some electrons left in this semiconductor area. With the input signal to the gate you can increase the current from source to drain (output signal) as well as decrease it. If there are a lot of positive charges on the gate then it will attract a lot of electrons and the current flowing from source to drain will increase.
You can try to change the charge of the gate by modulating the knob and see what happens.