When replication is running the volume is NOT locked down in anyway.   A snapshot, which isn't visible to you is created and that's what is being replicated.  It's a pointer to blocks as the volume was at the time the replication was complete.  At no time are new writes delayed to the original volume, the array never has to 'catch up'.   otherwise performance would suffer.  Especially since replication can take hours, days, weeks, depending on the amount of data and link speed/latency.   The local reserve is where you store the bits before the change.   The new writes go to a new page.   When you keep the failback, that last snapshot is preserved so that you can more quickly replicate back from the DR site.  So if you bring up the DR site and modify 100G out a 500G used volume, only that 100GB needs to come back to the primary to get it back in sync.  Otherwise the entire 500GB of allocated data must be re-replicated back to the primary site.   The only downside is that you will use up local space if you have allot of replication.   Typically, you leave free space and let replication 'borrow' space as needed during replication, then when complete it will return that space to the pool. 
So here is my understanding of this , so when a replication is going on it locks the volume down, so when any changes come into the volume it uses up local repication reserve, and consumes this space, and you can also check mark it (to keep a failback copy of these changes) which is essentially a Snapshot. so when the the replcation completes it releases this space , and if you can keep the (failback copy of the Snapshot you could use this if you wanted too. ) but if the volume is locked during replication, and local reserve is using this space , once replication is done on this Volume do the changes that were being used in local reserve get written to the Volume when it competes replication, becuase the the volume is locke ddurign this operation, so my thinking is that it can wrte these chnaged until replciation is complete. would this be correct?
Local Rep reserve