Hi Fishburg,
On my pilates DVD the guy on it says during the warm up to stretch out then press the back flat on the floor then bring the curve back to a place that is comfortable, kind of in between the start and flat position (hope that makes sence!)

Hi Hippie,
I love doing pilates! I have back and knee problems so needed something low impact. I've only been doing it for a couple of weeks but feel the difference already, I've always had bad posture and having to stand up straight always hurt my back but that has improved a lot. I have a lot of weight to lose so not really seeing any benefits yet but definatly feeling them. My advice is dig those vids out and get to it you'll feel the difference!