my loser back really arches in this pose. Should my tail bone be drawn in toward my knees and my belly strong? Where is the strength (aside from the thighs) coming from in this pose?

Not "loser"! 

Excellent question and body awareness, Kerry N ! There are different schools of thought here. I would say it is definitely okay to allow for the natural curvature of the low back/ lumbar spine here. However, many of us tend to over do it especially if we're hyper flexible. I would advise a strong lift of the deep low belly and a dropping of the tail bone down towards the floor. The strength is in the entire legs, the glut muscles and the lift of the core. Try this out and let me know how it feels! 

Thank you so much! Long delay b/c I was trying to do saddle pose with Alana and jammed up my lower back. I can't even get into chair pose. I'm so frustrated with myself. I've never had much lower back tightness so this is freaky. Any suggestions? 

Oh, I'm so sorry, Kerry N ! Injuries or even just temporary areas of contraction, etc can be so frustrating. My suggestions are anything that you feel will safely open the low back- both child's pose and a supine twist can be very helpful. Also, of course, go gently with yourself. ; ) 

Thank you--will do these. It seems that so little set off something so enormous.

I understand, Kerry N . And I hear this extremely often. It makes me wonder what lies beneath the surface- what has been there for some time, but then reveals itself in a more extreme way. It seems to be so common. Sending empathy and support for the healing. 

Thank you, Rosemary. I did the stretches you suggested and am now completely fine. I feel like the "injury" has shown me a little more about attention to subtle movement. I feel a little more affection and gratitude for my lower back. That's not at all something I would normally think or say so I wonder if that is a bit of what was "under the surface"--lack of a certain awareness/gentleness.

Kerry N-W , this is absolutely beautiful. I am happy to hear you feel completely fine. I am even happier to hear that the experience led to more attention to subtle movement as well as more affection and gratitude. This of course is not always the case. Brava to you for listening to your body and responding to the "injury" in such a mindful, gentle way. And for sharing the experience. Be well.