I do tend to warm up for five minutes or so but thought that seeing as I was defending the anytime, anyplace any......well you get the idea, then I should get up and prove it.
The video below then is without any warm-up other than just popping up the shops and laying out the Manduka.
So what does this mean? I'm no Susan, nowhere near as flexible, so it's not about flexibility. I didn't feel any strain anywhere, probably because the backbend is spread out. I'm not just bending my back. it begins with dropping my shoulder down my back , dropping back my head, my shoulders. Then quite a bit of a bend in the Thoracic, though not too much because I'm aware that I haven't warmed up. A little bit of bend no doubt in the Lumbar region then it's pushing the hips forward, sliding my hands down below my backside and pushing there and even down at my thighs. All this before I raise my hands to my chest and give it the last little bit, oh and note too that my knees are quite far forward bending at the ankle.
My point is that if the bend is really spread out throughout the whole length of the body then i think you can do it with out the warm up. If you focus on any one area and try to over stretch it, then you probably will strain yourself. Of course at the end of a practice when I am warmed up, then I can of course make it all a little deeper and drop back from even closer to the ground.
Standing where I do I make sure that I can see quite a bit of the mat before I drop back. As long as I can see it then I'm confident to drop. In the end it seems it's more about technique, spreading out the bend, and confidence rather than JUST flexibility.