This slider controls the rate or speed of the main LFO.
The LFO outputs both sine and square waveforms. The Sinewave helps produce vibrato effects while the Squarewave is useful for things such as trills. The LFO Sine can be applied to PWM and the Filter, whereas the LFO Square can only be applied to VCO pitch.
The LFO Frequency also dictates the rate of Sample & Hold (S/H) modulation.
When set to the ON position, the LFO is retriggered each time a key is pressed. It can also be useful to switch this function OFF, particularly when the LFO is intended to slowly modulate the sound over a long musical passage. As such, changing the position to OFF allows the LFO to continue without being retriggered by a note-on event.
The LFO can be synchronised to the host tempo by switching this ON. With it ON, moving the pink LFO slider allows you to choose one of several LFO templates including: 1/4 beat, 1/3 beat, 1/2 beat, 1 beat, 10 beat etc...
In the OFF position the LFO rate is set using the pink LFO slider and its value can be seen in the Main Edit window in Hz.