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.
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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.