Questions about reacting to bodily sensations

I'm currently reading _The Art of Living: Vipassana Meditation_ and it says to observe sensations in the body without reacting. 1. What would be an example of a reaction that one might have? When I feel an emotion - perhaps I remember a conversation that I had that produced a strong emotion - I notice a sensation in my body associated with the emotion. Is the goal to focus on the sensation and wait for it to pass? 2. Also, I've tried starting a meditation session by focusing on my breathing. I then try to observe sensations in my body -for example, my stomach gurgling, a tingling on my skin or the temperature on my skin. If I focus on this, no emotions are coming up because I'm focusing only on my breathing or sensations in my body. In the book it says to observe the sensations without reacting. There's nothing to react to though if I'm just focusing on the bodily sensations so I'm not sure what this piece of advice is referring to.