from a Yoshinkan website (http://www.yoshinkan-aikido.com/soke.htm
"Chu Shin Ryoku", the 'centre power', refers to the strength required to maintain your body's centre line straight. If you don't develop this you cannot achieve "Shu Chu Ryoku", Concentrated Power or "Kokyu Ryoku", Breath Power.
If you don't develop 'centre power' you are just going through the motions of Aikido techniques without achieving true power. If you want strong 'centre power' you have to be able to develop correct "Kamae", Posture. You can feel your 'centre power' when you practice correct Kamae. So, at the same time as you practice Kamae, you learn how to strengthen your 'centre power'.
It is not easy to develop strong 'centre power'; it depends on how much you try. The key lies in the big toe of the back foot. The power comes up from the big toe of the back foot and is transmitted into the hips and the lower back. You have to develop strong big toes through practicing "suwari waza", kneeling techniques. You have to develop strong hips through practicing "Kihon Dosa", the basic movements. The techniques of Yoshinkan Aikido are those achieved by training yourself and making yourself correct.
[bold added for emphasis]
Maybe I am being rather naive, but Shioda really does break it down in somewhat easy to understand terms.
When talking about center power, is this the same thing as developing haragei? Just need clarification.