Low inter-temporal rate of substitution in the good state of economy

I am very confused about the analysis given by textbook p409 that the inter-temporal rate was lower in the good state of economy compared with the bad state. According to the illustration before, in a good economy, investors tend to consume more today, thus, leading a relatively lower current marginal utility and a higher inter-temporal rate. Could someone help to explain the exact relationships, especially the textbook’s analysis?