Can I read anywhere who hook.io distributes events?

[K.]

Does the "main hook" make a "broadcast" for every event or just a "multicast" to every hook that called the on() method for this event? 

[Jeremy Darling]

If you take a look at the hook.io source code the answer is yes.  Basically it depends on the event type, but in general, a multicast for each event is sent only to registered listeners.  Under the hood the OS may decide that it is more appropriate to perform a broadcast rather than a multicast.

 - Jeremy