ES P features an LFO (low frequency oscillator) which can do either of the following:
Modulate the frequency of the oscillators, resulting in a vibrato
Modulate the cutoff frequency of the dynamic lowpass filter, resulting in a wah wah effect
Vib/Wah knob: Turn to the left to set a vibrato; turn to the right to cyclically modulate the filter.
Speed knob: Set the rate of the vibrato or cutoff frequency modulation.