I can't think of a non-bossy way to say it, so I'll just put it this way: You have to learn the pelvic curl exercise*. If you already know the pelvic curl, you need to review it. That's how important it is.
If you get the pelvic curl, you will have the critical start on the all-important spinal articulation exercises in Pilates like the many roll ups and roll downs from the mat to the reformer and other Pilates equipment.
Pelvic curl has more great benefits as well; like training you to sequence movement through your spine, to rotate your pelvis differentiated at your hip, to engage your glutes and hamstrings, as well as to stabilize the shoulder girdle, open the front of the hip and many more. Pelvic curl is also an abdominal muscle strengthening exercise that helps lengthen and support the low back. It is often included in Pilates exercises for back pain.
Get Detailed Instructions for Pelvic Curl
Learn Modifications for Pelvic Curl (video)
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*Some people shouldn't roll up onto mid-back/shoulders. If that is the case for you, you can still do the pelvic tilt that is the first part of this exercise. See the instructions above.
photo: (c)About.com

I find all the advertising you allow on your blog totally offensive …. I sure hope you are making a stack of money out of them !!!! Most have absolutely no connection to Pilates or its benifits… I too have a blog, and have turned done all irrelavent advertising… Please stop the fat lady flashing at me while I am trying to read your instructions.
Thank you.
Keep in mind all the valuable information from Marguerite and About.Com Pilates is free to the subscriber – the rest you are free to ignore.
It’s true. The ads do make it possible for us to deliver everything for free. I’m sorry they are sometimes obnoxious. If one is really, really bad, you can report it to customer care. As the guide to Pilates I really have nothing to do with the ads or the choice of ads.
Thanks for reading!
Marguerite, the Guide to Pilates
Wow that was strange. I just wrote an really long comment but after I
clicked submit my comment didn’t show up. Grrrr… well I’m not writing all
that over again. Anyhow, just wanted to say fantastic blog!