What exactly does "relaxed" mean?

A few weeks ago I posted this question in a Facebook group devoted to triathlon coaching:

I was on my TT bike doing a turbo session and found myself studying what muscles I was using to hold aero position. Hold with the back? Hold with the chest? Relax as much as possible? Stay stiff like in plank position? Then I realized I have never seen any discussion about this, just get out and ride. That and get a bike fit. Is there a "Everything you wanted to know about riding aero but were afraid to ask?" I cannot say it comes naturally.

My post generated some interest, but little advice. Someone said to not use back muscles, rather use core. I find "core" to mean different things to different people. To many it means the abdominal muscles, i.e., the belly. 

The group was started by someone I think of as a good friend even though we have never met -- Suzanne Atkinson. She replied, "The posterior muscle group is just as important as the other muscle groups in all things tri including aero position. So the back is important, as is the core."

(Posterior means towards the back. In this case, the muscles on the backside of the body. In particular the calves, hamstrings, gluteus maximi, latissimus dorsi and the erector spinae.)

Having nothing better to do I happened to be looking back through Going Long, by Joe Friel and Gordon Byrn, and came across some relevant material in the chapter called Training for the Bike. The first passage is from the section on developing pedaling economy: 

There are two keys to improved economy. One is to refine the movements that take place at the top and bottom of the stroke, when the leg must shift from "up and back" to "forward and down," with the reverse situation at the bottom of the stroke. At the top of the stroke, the foot should feel as if it is pushing forward in the shoe. At the bottom, you should get the sense that mud is being scraped from the show.

The other key is to focus on the relaxation part of the pedal stroke. Tensing more muscles than are needed to apply force to the pedals is wasteful and uneconomical. Start with the muscles you have the most control over -- the face and fingers. Once you can relax them, try relaxing your calves and toes while spinning. The idea is to activate only the muscles needed when they're needed. No more, no less.

The next is from the section on developing cycling skills, specifically a fast cadence spin-up drill.

The key to this drill is relaxation. Relax your toes, your grip on the bars, and your face. Make it seem almost effortless.

These are just a few of the many times I have seen relaxation described as essential to success, nowhere more than swimming. I chose these passages because they relate specifically to my question about cycling.

Not long ago I purchased a book on triathlon (TT) bike fitting by a regular in another forum. In fact, I believe he created it. The Misfits Guide to Basement Bike Fitting, by David Luscan. A major theme of the book is that our body is capable of working efficiently all by itself. The key is to get our over-active mind out of the way. 

Here is something Dave wrote about posture, specifically when settling into the aero position:

[The rotation of our body] is centered around the hips, and the technical term is anterior pelvic rotation ... We don't slide to move forward, we roll. Then we relax. Like a drunk slumped over a bar, we want to forget everything we think we know about proper posture, and literally slouch down into the bars.

Colorful, and evocative. Later on in a section on the cockpit he writes:

This fundamental fit coordinate is the straight line distance from the tip of the saddle to the middle of the back of the elbow pads, and it is the primary driver of comfort on the bike. Too long or too short and we end up using too much muscle for support. Just right and we can execute postural directive #2 -- Relax.

I never did find any more postural directives from Dave, but that's OK. The key here is how these two passages connect through relaxation.

A yoga instructor once said that I could go further into a stretch only my muscles don't allow it. That if I were unconscious I could, for example, touch my toes. That image and the drunk slumped over the bar called up the movie Weekend at Bernie's. I am quite certain that if we put Bernie on a TT bike in aero position he would fall off. Even if we tied his wrists to the aero bars and his feet to the pedals.

Yesterday I rode the bike portion of an Ironman VR 70.3. By far the longest I have ridden this season, especially without a cafe stop along the way. I have made many improvements. I can see. A new and better saddle. Raised and shortened aero bar position. I would not go so far as to say I was comfy out there, but it was my best ride in a long time. 

As the hours ticked slowly by I found my thoughts returning to this question, again and again. Relax, but not so much I lose control of the bike. Where I ended up is best described by Friel, "The idea is to activate only the muscles needed when they're needed. No more, no less." He was writing about pedaling, but his words apply to, well, everything. 

That is what I call relaxed. Yes, we do use muscles to hold our position. A great many. But the key is to use the least amount possible, to feel as if you are not using any.