the script, when put in /etc/network/if-post-down.d/, will be called every time a network interface goes up or comes down.

if it wasn't called during a "stop" event, do nothing (exit script).
if the network connection is neither a WLAN infrastructure nor WLAN adhoc, do nothing.

finally, really bring down the interface (ifconfig down).
at this point the interface must be a WLAN being disconnected, because all other possibilities have been ruled out by the two "exit" statements.