Telepathy is used to advance the plot in the chrysalids what are incidents where this applies and what is the significance?

There are several examples, but three of them all involve David's little sister. The first two are when she isn't really old enough to know what she is doing and she, very loudly, expresses psychic distress and calls David and others to her for assistance. The second incidence of this is what gets them found out, actually. The third time is when she and David are in the process of running away and she starts communicating with the woman from Zealand, who is able to eventually rescue them.