There three stages to learning the Aikido techniques. Stage 1) Rote: This is the stage where the physical form of the techniques are mastered. Students spend their days toiling away at what seems like an endless amount of repetitions where the only encouragement given by the teacher is "more." There should be no thinking at this stage because the technique is being learned by the body. At this stage the movement is mindless.
Stage 2) Reaction: After rote mastery is established, the movement becomes "second nature" and the student's body begins to move with a lightning quick reaction to the corresponding event. It is at this stage that the student begins to develop "intuitive" movement where the brain reacts to the event with such a quick reflex that it almost seems inhuman. Here, the practitioner moves before they have a chance to think and thus the movement is still mindless.
3) Action: After years at the rote and reaction stages, the mind begins to develop what I like to call an expansion. In professional sports this expansion is called "being in the zone." When one is in the zone, time, space and movement seem to expand and slow down. This expansion enables the high level practitioner to only observe the attack. By only observing the attack there is no "reaction." By only observing and not reacting, the student can then act appropriately. When they can act appropriately then the the movement is mindful.