how to use watchdog in linux

In the boot log, i saw "adi_wdt: initialized: timeout=20 sec (nowayout=0)",

but how to test whether the watchdog is really work to protect my system?

and in the /etc/init.d/S15watchdog file,

start)
echo "Starting watchdog..."
watchdog -t 5 /dev/watchdog

i changed it to 

watchdog -t 5 -T 2 /dev/watchdog

which means it shoud reboot after 2 seconds if no reset,

but it did not reboot , which confused me, how to test the watchdog?

thanks