Vibrato knob: Rotate to set the amount of vibrato (pitch modulation).
Sync knob: Set the maximum amount of sync modulation. This makes the sound more or less aggressive. Technically, this control changes the waveform start point of both oscillators.
Mix slider: Move to crossfade (set the level relationships) between the oscillators (Shape 1 and 2).
Download the guides:
Logic Pro User Guide: PDF