From Famitracker Wiki
This page regroups effects that can only be used on the DPCM channel. Using those effects on other channels will do nothing.
Wxx overrides the DMC sample speed set in the DPCM instrument window. Valid range for xx = 00 - 0F.
Xxx retriggers the sample on the line it's used on. xx = delay in frames between each retrigger.
Yxx adds an offset to the DMC sample. The larger xx is, the longer the offset will be. Each step adds 64 bytes to the offset.
Zxx lets you write directly to register $4011 of the NES, which changes the DPCM's delta counter directly, thereby affecting volume of the noise and triangle channels simultaneously. xx is a 7-bit value, and therefore must be between 00 - 7F. 80 - FF correspond to the same values.