...clipped
In other words, we don't necessarily always work on the same block until it is solved(but we might solve it and get the coins), nor do we work on a "new" empty block until it is solved(side note: I originally guessed this was what was happening - we decrypted dummy data in the new block then we would be handed a "real" block to move into our new solved empty block, stripping out the loot in the process.
[edited to be more clear]
That is kind of what I thought was going on...and was wondering why we as a pool just didn't toss the block we were working on and picked up another one...
I'm pretty sure the pool switches to the new work as soon as a new block is found. I don't think there is any "dummy data" work being done.