Resilient Channel and Drywall Orientation
Perpendicular. 

I found this 

"Resilient channel should be installed on Cathedral Ceilings (single or double angled ceiling) with the center of the face of the channel 6 inches from the top to permit a floating angle wallboard installation. Resilient channel must be installed with mounting flange down."

at the following website. 

https://www.wconline.com/articles/89218-preventing-ceiling-cracking-with-resilient-channels

And of coarse much info is available on YouTube such as this YouTube vid.