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.

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?

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.
an example of a typical reaction would be bodily feelings of pain and reacting with aversion, anger, etc. Or feelings of bodily pleasure, and then the mind running off with thoughts of things from the past that the bodily pleasure reminded them of.
what you describe is exactly what should happen if you're doing breath meditation correctly, or kaya-gata-sati body immersed mindfulness correclty.  You're focused on the kinaesthetic experience of how the breath and bodily sensations feel in all the cells of the body, so the bandwidth of your attention is completely occupied with skillful thoughts of Dharma, and there's no room for unwanted thoughts and emotions to interrupt. This is training of samadhi, training the mind to think what you want to think and not think what you don't want to think, using the breath as a skillful way to occupy the mind.

Now where your question #1 is ultimately heading, building on #2, once you have stability and control of your mind, then you can observe sensations in the body without the 'reactions', typical deluded responses and filters we impose on top of the raw experience of mind and body, and instead use lucid discerning and wisdom to see reality moment by moment as it actually is, and by doing that, you free yourself from suffering.