I have pulled the Boeing book from the shelf, B 737-300.
Center Tank Scavange Pump
"When both center tank fuel pump switches are turned OFF, the fuel scavange shutoff valve opens. this allows fuel pressure from the main tank No. 1 forward pump to operate the center tank scavange jet pump, which transrefrs the remaining center tank fuel to main tank No. 1. Ater 20 minutes, the fuel scavange shutoff valve automatically closes."
Which is how I have alwaysbelieved it to be, I have no knowledge of the NG so perhaps someone can come up with a Boeing quote for that?
