Traditionally when you're passing a knot with a casualty, there is an element of "shortening" one of your ropes, either to re-anchor your main-line to itself, or by offsetting the knots to switch ropes.
BUT, what if your ropes are already too short? If you offset them further you won't reach the ground with your casualty. The easiest solution is to make sure you have long enough ropes, but JUST in case, we can actually "park" our casualty back on the ropes, separate ourselves and pass the knot one at a time. Watch Cam, the man, show you how it's done!