You got a partial answer. If "SKIP" was ever used, I'm interested to know by whom. I only know of a single example, which was EXTREMELY special case.
The commands:
MOVE: move immediate value into copper register. These included such actions as where in memory the video memory was (i.e. - where to fetch the picture from), what is the palette, video resolution, where on screen the video fetch starts and where it ends and many others.
WAIT: wait for the screen update to reach a certain point. There was a mask argument which allowed you to have don't cares on some of the bits.
These two was how most of the programs worked. From simply displaying a static image (the memory fetch registers had to be reset each frame), through "copper waves" (things like telling the hardware to start displaying the image in a different timing each line, so that a straight in memory was a wave on screen), to what matfud erroneously called "display HAM in a window" (it took several scan lines for the copper to completely replace the display mode, so you couldn't display two modes side by side, but you could display them one above the other). It also allowed "virtual sprites" by reusing the same 8 hardware display sprites for different things in the same frame, so long as they were not in the same line.
The third command, skip, had the same arguments as WAIT. Instead of waiting, however, it skipped the next command if the condition was not met. Add to that copper registers that restarted the copper program if written to, and the fact you could load two start addresses and switch between them, and you get the ability to perform a loop.
Back in 1994 there was a Mac "emulator" called A-Max. It was not really an emulator. It loaded Mac ROMs into the ram, patch some hardware related entry points so that it would work on the Amiga, load a copper instruction that caused the Amiga display to act like a Mac black & white display, and simply executed the ROMs. As a result, a 7.2Mhz Amiga 500 ran programs written for the 8Mhz Mac at 120% speed.
One (rather minor) problem they had was that while the display content was showing correct colors, the Amiga was hard-coded to use color 0 for the overscan. As a result, the overscan, black on the Mac, was white on A-Max. Around 1993 I figured a way to resolve this, using the SKIP command and the loop method mentioned above. I assumed that if I figured it, it was obvious, and didn't do anything with it. Around a year or two later, A-Max released a version which had text similar to the following:
Thank you (don't remember the name) for providing us with a method to give a black overscan without writing a huge copper program. As thanks, we've given him a free version of A-Max.
Which caused me to kick myself no ends, and never assume my ideas aren't innovative.
As far as I know, there was no other use for the skip command (which might explain why the A-Max guys never thought about it themselves).
Shachar