Source: Code: The Hidden Language of Computer Hardware and Software by Charles Petzold
Despite the fancy appearance, this was just a switch designed for maximum speed. The most comfortable way to use the key for long periods of time was to hold the handle between thumb, forefinger and middle finger and tap it up and down. Holding the key down for a short period of time produced a Morse code dot. Holding it down longer produced a Morse code dash.
First of all, why do you think there is no definite article at least before the word thumb? And secondly, how do you think the sentence would be different in meaning or style if we had placed one there? Would it be grammatically plain wrong or just constitute a bad writing style?