It amplifies from the 47kΩ resistor that amplifies through the NPN transistor so it's more sensitive to smaller voltages. When it amplifies for the negative direction, it ties up to the 820Ω resistor(0v input), when the wave is higher(+1v), it's lower resistance, and when the wave is lower(-1v), it's higher resistance from pulling the 47kΩ low amperage down from base to emitter.
