Think of the derivative of the red curve. It represents something like “for each marginal person who switched their behavior, how many total people would switch after counting the social effects of seeing that person’s switch”. If the slope is less than one, then small effects have even-smaller social effects and fizzle out without a significant change. If the slope is greater than one, then small effects compound, radically shifting the overall expression of support.
Think of the derivative of the red curve. It represents something like “for each marginal person who switched their behavior, how many total people would switch after counting the social effects of seeing that person’s switch”. If the slope is less than one, then small effects have even-smaller social effects and fizzle out without a significant change. If the slope is greater than one, then small effects compound, radically shifting the overall expression of support.