function foo(required:String, optional:String="n/a"): void
{
        trace(required + ", " + optional);
}

If you use set numArgs to 1, only one argument will be passed and optional will be defaulted to “n/a”. If you set numArgs to 2 but only pass one argument, optional will not be defaulted but instead set to what you passed: undefined (although this will be converted to null as Strings cannot be undefined). If you set numArgs to 2 and actually pass two arguments, it will be as though you simply called the function with those two arguments. If you set numArgs higher than 2, you will get a runtime error (the same one as in the article) for passing too many parameters.

This last case is the subject of your second question and is answered in the comment for the reverse() function:

        *   @param numArgs Number of arguments to allow through to the function.
        *                 This can be used to increase the number of arguments
        *                 passed to the function, but all arguments beyond
        *                 those actually passed will be undefined.

The reason for this is that there isn’t a convenient way for reverse() to protect against passing too many parameters since it does not know how many parameters the function takes. There may be some (very slow) way of finding this out, such as describeType(), but I’d rather not add such tremendous overhead to the function.

Thanks for the comment. It’s good to fully explore the details of using such a function. I hope you find it useful.

your (args.length = numArgs;) can be greater than the number of given parameters, is not a problem ?