I really liked this passage from a larger meditation on prayer, Pray without ceasing: a daily plan of attack.
. . . mental prayer is the goal. Vocal prayer is the training ground, the springboard which is meant to take us deeper into the realm of contemplation and meditation. The Rosary is, in fact, the perfect example of this. It is one thing to simply burn through the 50 “Ave’s” of a daily Rosary, in a purely vocal manner; it is another thing to enter mentally into the mysteries of the Rosary, and to pray them in a contemplative way. Ideally, the vocal part of the Rosary gets us into the “rhythm” of prayer; because the prayers are easily memorized and can be recited without too much mental effort, the mind is then free to conjure up the images associated with the mysteries.
The technique is difficult to explain, but the body and soul are capable of working simultaneously without collision; the mouth can be silently moving, forming the words of the prayers, while the mind is busy “seeing” the events of the mysteries, almost re-enacting them for the purposes of meditation. The fingers stay connected to the beads, and continue counting the prayers; a good Rosary will have smaller “Ave” beads, but larger beads for the Our Father and Gloria Patri, so that the mind is “awakened” from its meditation when the fingers sense that the Rosary decade has ended.
I think it’s the clearest explanation I’ve seen of how the rhythm of the rosary is meant to become rote. The thing to keep in mind is that as your mind wanders from the words of the prayers, you must direct its wandering. The mysteries are supposed to do just that, provide substance for the mind to chew as it drifts.