Neutral Back! Contract your abs!
Keep a neutral spine! Contract your abs! We say these two things all the time, but why are they so important?
A neutral spine optimizes safety; it keeps the back in the best position to remain healthy. A non-neutral spine, on the other hand, looks like either the picture on the left or the picture on the right. When your back over-arches or over-extends, the spine can be compromised. In fact, this is how herniated disks can occur! When one side receives more pressure due to the back's position, stress can cause the discs to displace.
Contracted abs, or "tight abs" as many like to say, adds to the safety of the spine. When the four sheets of abdominal muscles contract, they form a tight shield, in essence, around the spine. They hug it; they help negate the slipping and sliding of the discs. When your abs aren't tight, and you bend and twist, the spine is severely compromised. This is when injuries are more likely to happen.
So just remember: Neutral spine! Contract your abs! By doing these two things, spinal integrity is greatly increased!