Maybe it helps, when you know how the external flash is being triggered, as far as I know, there are several ways, to do this (everybody: please refine, if something is wrong):
a) The external flash works as a slave flash. When it sees the light of the camera flash, it is triggered. Problem: When the camera sends a pre-flash for exposure measurement. Then the slave flash needs a detection system, so it is triggered only by the second flash of the camera. (Else, it would be triggered by the first flash, when the shutter is not open yet.)
Further, the external flash has to see the camera flash.
b) The external flash is triggered by an IR-Signal, send from the camera. This is the common way for most of the Nikon flashes. The IR-Signal can also send TTL-information. Problem: The IR-sensor of the external flash has to see the IR-signal from the camera (That’s were the in line statement comes from I assume)
c) The camera and the flash are connected via a real wire/cable. This is a very reliable solution, but there is the cable alway around somewhere.
d) You replace the wire by a wireless connection. This works similar as c, is much more reliable than a) and b), works also over larger distances and when there is no visual connection between external flash and camera. Problem: you need a wireless set and additional batteries. And the batteries you carry with you are always empty. I haven’t looked into it but it seems one of the newest flashes from Nikon works with wireless?