This means any muscles, not just the big ones! So, for example, even learning how to relax your hands and fingers, or you facial muscles, can help conserve energy that can then be used for the run or climb or long-distance swim.
Here is the quote from Lynn's book:
Planning on a midsummer ascent, I started training in early spring. I ran and climbed nearly every day, increasing the intensity of my efforts on a weekly basis. Because I was preparing to free climb a thirty-three-pitch route, I needed plenty of endurance and a high level of power; the most difficult sections of the Nose begin after nearly 2,000 feet of climbing. As I practiced going the farthest while expending the least amount of energy, I discovered a new consciousness in my climbing.
I learned to appreciate how subtle shifts in my attitude could greatly affect the quality of my movements. By focusing on maintaining a "soft grip" and a "relaxed face," I was better able to relax all the muscles not necessary for each movement. By observing my breathing patterns, I discovered that while reaching in stretched-out positions, it was helpful to inhale in order to gain extra lift, and conversely, while making powrerful or dynamic moves, it was helpful to exhale air in a quick burst or to make a karate-style grunt. I focused on maximizing the use of momentum in order to move quickly through awkward body positions or to jump between holds instead of wasting precious strength. Conversely, I practiced minimizing all excess movement to arrive at a "still point" before committing to a delicate move.
Throughout the months of training, I practiced an attitude of acceptance; no matter what the situation presented, I made an effort to remain patient and relaxed each step of the way. My intent was to pay attention to my intuitive sense and follow the natural intelligence of the body. When I made this shift in emphasis, my whole approach changed.