Yup the in camera flash is what triggers the off camera flash. If you don't want the onboard flash to be visible then you need something like the Nikon SU-800 wireless flash commander which sits on the camera's hotshoe. Not sure if it's compatible with the Nissian though. Your other option are wireless transmitters.

Nikon wireless commander
http://nikonusa.com/Find-Your-Nikon/Product/Flashes/4794/SU-800-Wireless-Speedlight-Commander.html

Discussion on wireless triggers
http://nikonrumors.com/forum/topic.php?id=1833
http://nikonrumors.com/forum/topic.php?id=756

Hello, the built-in flash always fires to give the off camera flash a signal. The signal flashing doesn't affect the exposure but is visible if you shoot a mirror or something that give a reflection.

Hi,

I have just bought the Nissin Di866 flashgun. I notice my in built flash fires no matter how I set it in commander mode. I followed some online video to set-up the camera and flashgun to use it wireless, but I don't want my built in flash to affect the exposure.

my setting in the D90 commander mode
built in flash -- --
Group A TTL 0.0
Group B -- --
Channel 1

setting on flash
-set it to remote

When zoom my lens, the flashgun does automatically adjust the zoom range. However, the built in flash fires whenever i press the shutter and of course the flashgun fires as well.

From what I read from internet, the built in flash won't be active if i set it to "--". It doesn't happen to me, it fires every time... :(

is it normal ? is there a way to deactivate the built in flash and only the wireless flashgun fires?