Signal emission is the process whereby GTK runs all handlers for a specific object and signal.
First, note that the return value from a signal emission is the return value of the last handler executed. Since event signals are all of type GTK_RUN_LAST, this will be the default (GTK supplied) handler, unless you connect with event#connect#after.
The way an event (say "button_press event") is handled, is:
Start with the widget where the event occured.
Emit the generic "event" signal. If that signal handler returns a value of TRUE, stop all processing.
Otherwise, emit a specific, "button_press event" signal. If that returns TRUE, stop all processing.
Otherwise, go to the widget's parent, and repeat the above two steps.
Continue until some signal handler returns TRUE, or until the top-level widget is reached.
Some consequences of the above are:
Your handler's return value will have no effect if there is a default handler, unless you connect with #event#connect#after.
To prevent the default handler from being run, you need to connect with #event#connect and use GtkSignal.emit_stop_by_name - the return value only affects whether the signal is propagated, not the current emission.