The forward ball roll from the knees helps sort out back ache. Done well, the exercise feels fairly effortless. Done badly and the abs burn, the back may ache and the shoulders hurt.
As Paul Chek says, maintaining the length of the spine throughout the exercise will properly engage the abs. The two main faults to watch out for are sticking the bottom out or flaring out the front of the ribs as you roll the ball away, either of which will make the low back bend and encourage the back muscles to get over excited.
We also have to check that the head stays in line with the upper back instead of dropping down. Life style – sitting at the computer or desk, driving, pushing a buggy – leads to poking the head forwards, which shortens the back of the neck. This can lead to spectacular headaches, stiff shoulders and general discomfort. When teaching this exercise, I find I have to encourage people to look down their front whilst keeping the head up to get a good head position.
Finally the arms and legs move together rather than one leading the other.
So the back of the body feels long from crown of head to the knees. There is no breath holding or jaw gripping. Which nails an important goal in strengthening the abs: if the abs stiffen so we can’t breathe properly, how are we supposed to exercise with a good level of back protection? Any sport, from running, cycling and all of the field and racket sports, done with floppy abs will result in reduced performance and a great likelihood of rolling about on the floor clasping some body part in pain or being unable to get out of bed because of back ache caused by an injudicious sprint yesterday. Also it is impossible to do any of these sports whilst holding the breath. There are brief moments of breath holding during a split second of the greatest exertion. If the abs are working properly, the brain will organise this naturally. And so done well, the forward ball roll feels fairly effortless which does not make it a valueless exercise. Just the opposite.