Wiring Gas Caps on Bombers?

Hello all. I have been reading up on my Great Great Uncle's memoirs from when he served in the Air Force during WW2 in the South Pacific. He details the 40 bombing missions he flew in a Liberator and often mentions "Wiring down the gas caps". Does anyone know what this means or why they had to do it. In one mission, he forgot to do this and didn't get to count it toward his mission tally because of that.
Have a good day.

 

If the cap came off, the slipstream would create a suction that could drain the tanks dry in a matter of minutes.

Not a good thing to have happen flying over the vast expanse of the South Pacific.