Neutral pelvis is when your hip bones (ASIS- anterior superior iliac spine) and your pubic bone are in line with each other and neutral spine is when you have the natural and balanced curves in your spine.  We often ask our intermediate and advanced clients to maintain neutral while doing supine ab work (laying on your back). You have to have enough core strength in order to maintain neutral without putting too much stress on your lower back. If you tend to feel pain in your lower back while doing ab work then you will want to work in imprint instead. Imprint is when you press your lower back into the carriage taking the natural curve out of the spine and strongly engaging the abdominals.