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FamiTracker
The delta counter works like this: when a sample is playing, the bits in each sample byte either increases or decreases the delta counter. If the counter reaches the bottom or the top it'll clip. The delta counter is then just connected to the DAC in the APU.
Changing the delta counter sets the starting point for the next sample, but there's one other side effect: the DAC is mixed with the triangle and noise channels in a nonlinear way. A high value of the delta counter will reduce the volume of triangle and noise. This can be used to slightly change the volume of triangle.
The delta counter ranges from 0 to 127, so 0-7F is valid for the Zxx effect.
All you need to do is play a steady note in the triangle channel & run the Zxx command in the DPCM channel. You should notice what JSR said; the triangle wave should clip from the top & bottom.
You'll notice a volume decrease in the triangle channel as you increment from Z00 to Z7F as a result of the wave clipping.
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There are relatively few uses for this aside from the triangle/noise volume thing mentioned above (I use this mainly to maintain volume consistency in a song that has parts with DPCM and parts without). One thing you can do is produce 'pops' that will sound kind of like a bass drum -- Final Fantasy III does this for its victory theme -- but from what I understand this only sounds good on the original machine, not most emulators. (Not having heard it on the original machine, I couldn't tell you.)
Frames 00-03: How to use a stream of Zxx effects (of course, in preferably very high tempo) to create attacks for the triangle channel. Yeah...
Frame 04: How to use only the command itself to make a buzzing sound.
Frame 05: How to use a special DPCM sample and Zxxs to create something similar to a sawtooth. The sample is just a 16 bytes sample filled with 00s.
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