It's quite interesting - this isn't ethernet as we know it. Instead of each NIC using its own free-running clock, all the physical layers are sync'ed to each other at layer 1. (note that gigabit ethernet, which is what it uses, sends data at all times - when idle it sends the idle symbol)
In short, it's about giving PTP and SyncE some extra smarts.
Since cm precision is often not possible, is roundtrip-length an estimated average from prior roundtrips?
jitter kills
Sub-nanosecond synchronization is getting into the relativity is measurable realm.