To explain what I'm talking about, suppose you type the word "prelude" but you meant to type "please". Realizing your mistake at once, do you:
- Remove the "relud" and insert "leas" in its place?
- Erase the whole word and type "please".
Whether or not you touchtype, you're probably aware that it's faster to retype the whole word. I've often tried to save time by correcting only the wrong letters. The minor problem is that you waste time thinking. The MAJOR problem is that you probably have a muscle memory for the entire word "please", it just flows from your fingertips spontaneously. There's nothing spontaneous (for most of us) about typing "leas".
I think about this when I listen to virtuoso pianists. They play notes much too fast for their brain to guide their hands to the next note. (Scientists have studied this effect.) Instead they seem to unleash long muscle memory pipelines to plan ahead where their hands will go. We do the same when we type.