Curl ups or sit ups are a minefield. Personally I don’t think they are the worlds best exercise, there are far better exercises that you can do to work the abdominals and curl ups are pretty hard to perfect. However they are used in a lot of Pilates moves in conjunction with leg movements so it is good to get the technique correct.
Your upper abdominals should be used when you are doing anything in a curled up position in Pilates. To help you use these properly and not strain your neck you need to get the correct neck alignment. Start with a small chin nod, and then use your upper abs to curl up, never lead with your head or neck when curling up, instead think about leading from your breastbone with your head coming up second. Support your head in your hands with your elbows just in your line of vision. Always keep enough space for a small orange between your chin and your chest. You shouldn’t feel too much stress in your neck. It is certainly important to have a strong neck but not to the point of causing pain or any additional tension.
Instead of crunching the abdominals focus on lengthening through the spine and keeping the waist stretched out. You want to curl up growing taller and then release the spine one segment at a time back to the mat.
You can always modify those exercises by keeping your head resting on the mat. Another helpful prop for people who are prone to neck issues is to roll up a towel to recreate your cervical spine curve.
(Thanks to Jo Fleet for letting us use this photograph).