The hbd that doesn't get staked is freed as the rc's hold comes off?
That's correct!
Once freed from the rc hold, those hbd could then be staked from the rc's earned by the staked hbd?
The RCs are generated restoratively. So when you have 1000HBD you have 1000HBD worth of RCs even if you received all that HBD 5 seconds ago. Once you use those RCs that HBD is dynamically locked/unlocked as it's used or regenerated. So if you use that whole 1000HBD worth of RCs, that entire amount is locked up until it's regenerated (or partially unlocked as those RCs become "freed")