Can someone explain this please?

I have seen a couple of vids on youtube, by 2 different people, yet saying the same thing. That being, that (person1) concerning the D3100 pop up flash which you can't use in commander mode, but they said they use it in manual mode to trigger an off camera flash, ie SB-700.

(person2) said he uses a D90 and uses the pop up in manual mode to trigger an off camera flash - again a SB-700.

Can someone explain the settings I need in both camera (D90) pop up flash, and SB-700 both in manual mode.
I have been tinkering but can't seem to do it other than of course in commander mode in camera set to --, and on remote on the flash.

I have checked the manual and can't find anything like this and didn't even know it could work.

Do I set pop up to M 128, and then channel 1 to say 128 or something like this, and if so what settings on the off camera flash?

I'm lost...

If the SB-700 has the same menu settings as the SB-900, then you need to go to the menu on the flash and choose SU-4. Then your camera's flash will set off the flash because it (the sb-700) becomes and "optical slave". Because any flash will set it off (including somebody else's), it seems that you could still use TTL on the built in flash. But that may not matter because everything on the "slave flash" has to be manual setting.

Was curious to see if it worked and it did. Thanks Helene.

Thanks for that ohkphoto

Helene is correct. Here is a sample of the camera operation is SU-4 mode using a Nikon SB 800


FujiSB800_002 by gqtuazon, on Flickr

Tried a few things with this set up this morning. Used my 3rd party flash on my camera and was able to get great lighting shots on my daughter.

You're welcome, gentlemen!

and Glenn, how clever of you! That photo should help.