The blastr code is very straight forward. It queries nostr.watch API for online relays and simply sends the notes to them using queues.

Any additional propagation is people running streams between relays (very common, even by non relay operators it seems), negentropy syncs, and that the popular clients when reacting or commenting republish the referenced note to their relay list (which may include blastr). I would be curious to see how much of nostr has blastr in their relay list, but not curious enough to get off my couch.

Overall, with the amount of my events moving around to relays I don't know about, I have to use blastr periodically to set the record straight on important stuff like relay list, profile. Or many clients that use hardcoded relays or search relays get confused, and likely so does anyone wondering where to find or zap me.