I've been beating on google trying to get it to answer this question for me and I'm not having any luck

What's the difference between an Ilex Acme Synchro and a Ilex Acme shutter?

Clearly, it has something to do with flash synchronization but both shutters appear to have bi-post so even the non-synchro shutter must be able to sync with flash.

What's the real difference?