He's on the right lines. What he's trying to describe is how to cause the system water to hang in suspension.
If it's an open vented system, cap the vent and bung the feed. Drain off water from the drain off until no more flow, shut drain off, close the rad valves conscerned, now you can cut and repair the pie and you won't lose a drop, not one drop. No need to panic or to hurry.
If a sealed system, close dust cap on auto air vent, ignore the open vented part, and otherwise as above.