From Famitracker Wiki
Duty cycle is the fraction of time that a system is in an "active" state. The duty cycle of a square wave is 0.5, or 50%. Some music synthesizers, including square channels of 2A03 and VRC6, can vary the duty cycle of their audio-frequency oscillators to obtain a subtle effect on the tone colors. Lower duty cycle makes sound sharper and thinner.
2A03 supports four duty settings (12.5%, 25%, 50% and 25% inverted). VRC6 supports eight duty settings.
The effect for changing duty cycle on the fly is Vxx.