Due to the weak couplings of the Qt signals and slot mechanisms, it is viable to bind signals to each other. It may sound confusing, so let me draw a diagram to make it clear: When an event triggers a specific signal, this emitted signal could be another event, which will emit another specific signal.
As Andrei pointed it out, signal are only a redefinition of protected, meaning they can only be emitted by the class in which they are defined. Qt signals are public in the sense that any object can connect to any signal.