Other way to see this is that "holding the pen correctly, or upright as was pointed out, the letter will be legible/readable.
That is, the reading will be (made) dificult (than it needs to be ), unintelligible of sorts. In child parlance, "it´s like a part of the letter is missing." or you could say "not well formed".
One aside note, is that writing is also a form to check damage to the brain in adults; of course, not the ultimate way, but it helps.http://www.dana.org/Cerebrum/Default.aspx?id=39304