Regardless of the cause, whether voluntary or involuntary, sucking the stomach in places greater pressure on the lower back and on the neck. This is because they now have to compensate for the changes in the core stability. The compression of the abdomen also reduces the amount of space that's available for the abdominal organs to reside. If you consider the abdomen to be like a tube of toothpaste, squeezing it in the middle creates pressure at the top and at the bottom