"Co-Auditing" means to pair up with another person, do each course together — Level 0, Level I, Level II, III, IV and NED — and audit each other back and forth all the way to CLEAR. This is not just a way to move up to Clear by your own bootstraps, it is actually the BEST — no kidding — way to get up the Bridge because of all the knowledge you will gain as an auditor. This combination of training and auditing is called a "co-audit" and a person doing that is called a "co-auditor."
Ultimately we all want to be OT — in other words, we all want to be able to live as effortlessly as possible and succeed. Scientology knowledge plays a HUGE roll in that.
But there is another factor as well: demonstration of competence. When you take someone else up through the Grades — which handled the Super Barriers to living — and make that person able to conquer those barriers one by one, the knowledge that YOU did that changes you like nothing else. This is "rewarding" on steroids. Your life is flooded with meaning. That, again plays a direct role in your "OT"ness because from that day on you are a victor over life. You will never, never, never be the same again.
Co-auditing IS the way to go. Even if you can afford professional auditing, co-auditing is better. It's always been better. And it will always be better. It will make you a better Scientologist than any other route. It will make you FEARLESS and LOVING. And there is no better combination than that because you can thereafter go anywhere and fear nothing and rescue anyone.