When you type faster than usual, it forces you to start looking ahead at the words that are coming up so you can figure out where to place your fingers in anticipation. So, if you see that the next four letters will all be typed with fingers on your left hand, you can move the correct finger on your right hand into place for the fifth letter ahead. Tests on the best typists have shown that their speeds are closely related to how far ahead they look at upcoming letters while they type.
— Peak: Secrets from the New Science of Expertise
by K. Anders Ericsson and Robert Pool