The unpressurised tip, with the car idling, came from Peter Remmington. His site is atNot heard this before - where did you get it from? I'd sooner run it unpressurized rather than with no coolant; that sounds a bit risky!
I 'phoned him to see about getting my spare MEMS box cloned to the one on my car whilst the car's off the road, and he gave me that tip, along with a couple of others during the conversation.
By the way, there is some very interesting stuff on that site, not least on diagnostic equipment and common ECU faults, including MEMS.