This Kiwi (New Zealand, typically with farmers, labourers and military) slang phrase is heard in expressions like "don’t bust your foo-foo valve", or "if you do that you’ll blow your foo-foo valve", when someone is trying to lift a weight or do a heavy physical task which might cause physical strain. It is used about both sexes. It has been mentioned that the "foo-foo valve" is the anal sphincter, so that foo-foo would perhaps be onomatopoeic for fart. Generally it is thought to refer to avoiding a hernia, a prolapse, or vomiting while under great strain and/or exertion.
For the past month Tom would hold his groin every time he sneezed since he had blown his foo foo valve while trying to lift the concrete wash tub into his van
Weightlifter Serge Blout'jagutski was urgently told by his friends to stay down in his squat position after they noticed a bulge and leakage in the rear of his gym tights that they presumed was his foo foo valve blowing out when he tried to lift the last weight.