The -c switch clears the flash status of all windows.
I tried confirming what I'm seeing with Wims, but he's not seeing what I'm seeing. Win7-32 using the beta 425. Using "/flash -c" is behaving as if it's stripping the 'N' parameter from any -rN that's active when "/flash -c" is used, but only if -c is invoked at the right spot in the blink cycle.
When /flash triggers for me while mIRC is not the active app, the icon in the taskbar does not blink. Instead, the taskbar icon flips to reverse colors and stays that way solidly. The icon is normally black text against a gray background, and /flash swaps the colors to instead be grayish text against a black background. If I hover the mouse over the taskbar icon, the popup icon having a red "X" for closing the program blinks instead, along with text "Message Waiting", but the taskbar icon in the row of icons does not blink.
To re-create: Immediately alt-tab to a different program after pasting this command into the status window:
//timer 1 5 /flash -r9 | timer 1 10 /flash -c
At 5 seconds, the 1st timer excutes, and the taskbar icon flips to reverse colors and remains solidly un-blinking while the popup window created by the mouse-hover blinks. But then at 10 seconds when "/flash -c" executes, the flash changes behavior as if the '9' were not used, and the icon remains black-on-gray 'forever'.
If I change the 10 into another even number which causes an odd number of seconds between the 2 timers, the behavior is as I described. But if I change the 10 into an odd number causing an even number of seconds between the timers, then the -c switch has the behavior I expected from the /help description, where the taskbar icon immediately reverts to the original condition prior to using any of the /flash commands.
I'm not aware of any other programs I use which will blink the taskbar icon on command, other than using "/timer 1 5 /flash" command in mIRC 6.35 and quickly alt-tab away from it. v6.35 blinks the taskbar icon itself, which the newest mIRC isn't doing for me. So this tells me that my Windows hasn't somehow disabled blinking of the taskbar icon.