Thanks for the follow up Russ. So, I tried my tests again and did indeed confirm that the YNU was firing correctly. Here's what I actually see as the sequence..
i. Pre-flash fires from camera, pop up flash ii. YNU flash fires during pre-flash iii. Wait for 2 seconds iv. YNU flash fires again at the end of the rear curtain. i'm using the YNU-560 flash, if it makes any difference.
Also, if I change the pop up flash from "--" to "M", the same behavior occurs. Thx, S