If my heart gets broken, it hurts. If I am living polyamorously and have other relationships, it just means there is possibly (though not necessarily!) more of a compassionate network there for me to find comfort in as I deal with the hurt. Poly doesn't automatically prevent me from feeling all that grief and heartache.
I think of rebounding as getting into another relationship right away to avoid that hurt and all the feelings of sadness surrounding the loss.
My heart breaks whether poly or mono. I can rebound whether poly or mono.
Rebounding technically means "bouncing back," which isn't always a bad thing - we can bounce back into living a full life after a period of mourning - but the term has come to be known as some kind of avoidance when it has to do with relationships. We've been taught that we never want to be involved with someone "on the rebound." But is it always something that will be disastrous if we do? No, of course not.
As with any human relationship, awareness is key to being able to handle the ups and downs, and possible consequences, whether a partner is collapsing from grief or pretending not to feel it.