Passive Backbend and Tucking the Tail

[Alex]

I'm a little confused about the cues when preforming a passive backbend on a wheel, roller, or bench. In both the Backbend Mastery Series and S&F its required for you to relax into the stretch once you have your upper back stable on the support. Is it implied that I should tuck the tail during a passive backbend? I'm feeling a good stretch from the bottom of my rib cage through my abdominals without tucking my tail. I imagine there has to be some stretch in the lower back during a backbend, though without causing any compression? 

I just feel like I'm not understanding the proper cues. If I were to tuck my tail and breathe/relax into the stretch, I'd lose the tail tuck as my hips travel downwards or when I take a deep breath. That and I feel like my abs are continuously contracting without allowing a backbend to take place.