If a client behind one of the vulnerable routers connects to an IRC server on port 6667 (and only 6667, does not DoS with other ports) and a user posts the following string in either a channel, private message, ctcp, notice, etc.. the router will drop the connection. The string is as follows:
DCC SEND anylongrandomstringhere
It is most commonly being seen as
DCC SEND "string" 0 0 0
but that appears to be unneccessary. The string does need to be 15+ chars in length.
Further, it appears the routers that are vulnerable to this are running vxworks as their embedded OS. Older linux Linksys routers appear to be immune.