The reaction of grabbing the ruler can be seen as a mechanical response to the stimulus of the ruler dropping.The time it takes to grasp the ruler can be seen as the time it takes for the brain to send the signal to the hand to perform the action.When the person focused their attention on the stimulus, the reaction time did not lessen.When the person focused greater attention on their own reaction when they were mentally prepared to grab before the ruler dropped - we saw that the reaction time was lessened.
When you're focusing on your own hand, the response, you are visually looking at your own hand, the thing that will eventually be doing the action of reacting. When you're looking at the ruler, you either have to take the time to change focus to your hand, or to rely on proprioception to tell you when your hand is open/closed. Using your visual sense as well as proprioception speeds up the reaction time.
