Found this great info on the web!

Neutral spine vs. Flat back

No, this isn’t a debate between contemporary and classical Pilates – I actually teach both spinal positions in my classes! I had a colleague ask me about pressing their back into the floor during Single Leg Stretch and Obliques, and another colleague ask me why she could only feel the Ab Series in her upper rectus rather than in her lower abs, and I think this may explain why you should choose one over the other.

It is much easier to perform a crunch with the lower back pressed into the floor rather than with the tailbone anchored and abs supported in a neutral spine, leaving space under the low back even in the top end of the crunch. Try it – but if your lower back grips up, it means you’re doing it wrong

Our bodies are designed to go in neutral spine. Our spines are curved to help us absorb shock, and learning to support your abdominals and spine without changing those curves is extremely beneficial. However, it’s also very difficult – the low back wants to grip up and arch further if you don’t keep your abs firmly pulled in!

I like to tell my clients that the initial ab engagement feels somewhat like the way you pull in your tummy when you’re trying to put on your “skinny” jeans that are a size or two too small for you to really fit into. It’s not extreme; in fact, it’s a fairly gentle contraction. This pulling in, or corset, feeling, comes from engaging your transversus abdominus, the second-deepest layer of your abdominals (after the pubococcygeus and levator ani in your pelvic floor). Once they’re engaged, they provide stabilization while your obliques and rectus work to counter your head, shoulders, and legs as they work against gravity.

If you’re new to exercise, start with a flat back. BUT, do that by pulling in and up on your abs, creating a scoop so deep that it causes your pelvis to tilt. Don’t get your glutes involved by squeezing them into a pelvic tilt. Then do your ab exercises. But when you’re ready to take it up a step, try working in a neutral spine. If your back starts to grip, start all over again – and really think hard about those skinny jeans!