FYI, I just got my new 622 and discovered the following.
When you watch a recorded show, the 622 deletes the buffer on the channel you were previously watching (as stated above). However, the buffer on the other tuner remains intact.
Try it yourself. Start with a full buffer on both tuners. Watch a recorded program. Stop the recorded program. The buffer on the tuner that comes on will be empty. However, if you hit PIP and then SWAP to the other tuner, you'll see that the buffer is full/intact.
The above assumes you're in Single Mode.