The Sine Level knob mixes a sine wave (at the frequency of oscillator 1) directly into the dynamic stage, independent of the filters. Even if you have filtered away the basic partial tone of oscillator 1 with a highpass filter, you can reconstitute it with this parameter.
In cases where oscillator 1 is frequency-modulated by oscillator 2 (if you have turned up FM with the waveform selector), only the pure sine wave is mixed into the dynamic section, not the distorted FM waveform.
Any modulation of oscillator 1’s pitch, set in the router, affects the frequency of the sine wave mixed in at this stage.
Note: The Sine Level knob is useful for adding warmth and a fat bass quality to the sound. Extra body can be added to thin sounds with this parameter, given that oscillator 1 actually plays the basic pitch.