"You can also create a PWM (Pulse Width Modulation) like effect by using a Tone's waveform (WG1) as an LFO to shift the other Tone's waveform (WG2) up and down. This parameter works best when you use it in conjunction with the Gain parameter (PATCH/WG/Wave Param)."
This effect works best when using a particular set of waveforms. I also believe the quote is backward. WG2 moves the WG1 up and down. It affects WG1 wave's DC offset. When the wave is higher it clips the top off the WG1 waveform. This changes the waveshape. If you use a triangle wave (or even a saw wave: say B26) for WG1 and a low wave (I like B189 Low Sine) for WG2, with the right settings for the gain, you will hear the WG1 waveform morphing. As the wave gets more clipped, it's perceived pulsewidth changes (a triangle is broader near the base than near the peak). This effect is similar to a trick they used to do with the old analog synths where they would use an LFO to modulate the pulse width of a wave (good for a chorusd effect without the chorus). Since the JV/XP's don't have pulse width modulation per se, they described (badly) this work-around in the manual. Hope this helps.
Jerry
PS: There is a better description of this effect in the supplemental notes PDF files available at rolandus.com