Lets begin with our good old actor Bruce lee and ascend to Buddhism afterwards....
What you are referring to is something called "Reflexes". In martial arts the reason they practice long hours is to train their bodies to fight automatically. This is the extreme level of fighting and he who is in this level is considered an elite. Here your muscles do the thinking for you and there is no need for logical reasoning (like your hand is feeling heat from a candle and immediately taking the hand away).
The Scientific explanation is very simple on this. There is a thing called "Muscle memory". Muscle memory is your muscles learning the things that you do constantly. This allows you to do things very fast and better than a novice.
for example :- Playing a guitar without looking at the strings, Typing something without looking at the keyboard and etc.
There is a common misinterpretation of what "Mindfulness" truly is, my school teach us differently and the form you are mentioning is "Not so effective" as to my teacher. So here is what we have learned as Mindfulness.....
Lets discuss your own example,
When i'm writing,like any other time my mind starts to wander and start many discussions within. I may even get angry or bored. I might even start to focus on the debate my mind has generated within a couple of minutes and jump into it. I am no longer focused on my writing and i may even write a wrong letter or a word.
Clearly here i have lost my focus and i'm jumping back and forth on my writing and debating with my mind. this is clearly not being mindful.
So if i focus on my writing and focus my thinking on what i have written and what i am about to write that focus my mind on the task at hand. Now i'm mindful about what i am actually doing this instance.
When asked about the version of mindfulness that is mentioned on the question my teacher had a reply like this....
(The person who asked the question asked......)
Is it correct to think i'm walking now, now i'm taking the right foot, now i'm taking the left foot. as a form of mindfulness?
(My teachers answer....)
What is the reason that we as Buddhists go on to practice mindfulness?
Because we are trying to reach Nirvana. And when we are not mindful the mind start to wonder.
Where does the mind wander?
- Mind wander in the adoring nice memories of the past, Bright hopes of
the future and in the moment we enjoy currently and it gets
- Mind wander in the cruel worrying past, dread for what future may bring and finds a reason to start hating right now
- Mind wander in a chaos. it jumps here and there, a moment ago you were laughing and now you are feeling bad and almost about to cry. Seconds later you are angry and a half an hour later you are burning in lust.
These three phases or sides of your mind feeds the craving you have for life,to be born again,to live forever,to chase the mirage of happiness.
This is bad because it's these three phases that drives a being further into "Samsara".
These 3 has brought you here and they will drag you through the dangerous lower realms to feed on innocent little animals,to be be hunted by lions and tigers,to be eaten alive and even to be tortured and burned in "hellish" realm. This is the true danger that a being face in Samsara. Because once there in a bad realm there is virtually no hope for a poor being.
And what about the good realms.....
Yes, those 3 sides of your mind also took you to "Deva" realm,to "Brahma" realm and here to "Human" realm. Not so bad you might say,won't you?
Well lets see. you have been born so many times in this samsara. Our Lord Buddha said
"Any being has been killed so many times.If someone collect all the blood that fell when he or she was decapitated,the amount of blood will be roughly equal to the amount of water in the sea."
Look at that old person next to you, after listening to the words of lord Buddha can you certify that he or she next to you has not being someone you loved?
Well lets see this in another way. We know what animals are,they are the poor beings who lost their way and ended up in a bad realm.
When you go home and eat a delicious meal like a fish curry or sausages,look at it closely. Imagine a one you had. Do you see your father or mother? How about that girl you promised to live eternity with in another life, don't you see her?
The sad fact is you do. Now you are eating them so greedily,just like they ate you. Look at this bizarre circle that we roam around from good realms to bad, from bad to good.
One day Lord Buddha smiled and there was Ven. Anandha thero nearby. As a smile from a Buddha is rare the venerable one asked why. Lord Buddha gave a strange reply, if you focus on this statement alone you can understand the true reason to practice "Mindfulness" and the true danger of Samsara. The answer was "Look Anandha, look at these ants here. There isn't a single ant in this long row of ants that hasn't been the king of earth".
Remember these words from Lord Buddha,
"Vinnana" is like a seed, attachment or craving is like water and Karma is the land.
Because your mind wander and get attached it creates Karma. Whatever you get attached to,you soon start to crave for it. There it is The land,the water and the seed, because of this you are born and will be born.
Practice to focus on the temporary nature of things and use your knowledge to see and feel the world not your body. practice to keep away from the 3 phases mentioned above.
Mindfulness Lord Buddha taught was keeping mind from those 3 phases mentioned above and maintaining your life and the very second you live in on 8 noble ways.