It's easier to explain in the IF where it's just reaction. See the pitch, lean in the right direction, catch the ball. In the OF, it's a little more difficult because you can get a bad read but, if you're fast, make up for it. I played several years where I was easily our slowest OF but covered the most ground. It was just better anticipation, more or less the ability to "read" the hitter, and start moving while everyone else was standing flatfooted. But, if I moved wrong, I was toast.