Let's start with buoyancy and trim. When I got to 30 dives, I thought, "Wow! I'm getting really good at buoyancy control!" When I had abut 80 dives, I thought "I was wrong about being good at buoyancy when I had 30 dives--now I have it mastered!" I had epiphanies of that sort every 80-100 dives. When I started tech, I realized I was still a beginner. I got better and better and better, but with well over a thousand dives now, I know people who still have me blown away. Want to test yourself to see where you are? The next time you do a 3-minute safety stop, do it in horizontal trim (perfectly flat), without going a foot shallower or deeper and without moving a muscle. Now imagine doing that for, say, a 30 minute decompression stop.
Now imagine doing that while accomplishing tasks like writing in a notebook, switching tanks, etc. By switching tanks, I may be talking about taking one that is trailing behind you on a leash. You need to take a tank off the leash, put an old, spent tank on the leash, clip the tank you took off the leash onto your D-rings, switch the regulator you are breathing from now to a different one on the new tank, and stow the regulator and its hose under the rubber bands on the old tank, all while holding that decompression stop.
Next, let's move on to propulsion. Imagine going through a wreck or cave with soft, fine silt on the floor and a hard ceiling with only a few inches higher off the floor than the total thickness of your body and gear. Imagine now that you glide over that floor and under that ceiling without touching the ceiling and its delicate formation or stirring up the silt on the floor. Imagine maneuvering your body through a complex, winding passageway, either in a wreck or a cave, using only the movements of your fins to turn your body without touching anything.