This "feature" exploits the fact that everything in options is a slash command. This particular option is a drop down menu with 4 settings. 0-4. If you were to choose -1 or 5, it would be out of range and the drop down selector would select nothing. EQ2 doesn't have a failsafe for that, so no particles are rendered at all. SoE didn't really intend for it(though they should have made it a valid option).
TL;DR
It still works for me with -1.
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