how do macro operator ## and ... work together in C

Question

I now trying to make a new flash driver for a specific chip which is not supported in openocd. Everything went well until I suddenly met the following code:

/**
 * Command handlers may be defined with more parameters than the base
 * set provided by command.c.  This macro uses C99 magic to allow
 * defining all such derivative types using this macro.
 */

#define CALL_COMMAND_HANDLER(name, extra ...) \
      name(cmd, ## extra)

I looked up in C Primer Plus and I know ... is for variadic macro and ## is for gluing. However, I cannot understand how it comes to the code I mentioned before.