The Extinction Rebellion have a very well-informed 45 minute talk which explains the purpose of XR in detail, it is called ‘Heading For Extinction and what to do about it ‘. About 8 minutes in there is a very clear explanation of climate feedback loops and tipping points. It explains how, for example, the continuation of forest fires in the Amazon means that the eco-system in that area will be changed to savanna rather than rain-forest. Therefore once this particular tipping point is passed, the remaining rain-forest will fail to survive in it’s current form.
Once tipping points are passed, there is a domino effect as the whole climatic system that has endured for millennia collapses. See slide below, the changes happen much more rapidly, as we are seeing in 2019/20.