I found this article online where a student is talking to Peter Ralston and asking about developing Chi. This internal source of power is meant to be used in Aikido. Here is a link to the article.
Art of War Blog
Read "The Art of Effortless Power" to start to get a handle on Peter. I have personally found his works interesting and inspiring, and have done workshops with both him and some of his students. They have influenced my aikido significantly. There are various aikido teachers out there who credit him with strong influence, for example Patrick Cassidy and Miles Kessler who both spent time at Iwama with Saito sensei. Have a look around the web!