A problem in icoFoam?
 

I read the source codes of icoFoam and pisoFoam. The codes in lines 108~113 of icoFoam is shown as following:
surfaceScalarField phiHbyA
(
"phiHbyA",
fvc::flux(HbyA)
+fvc::interpolate(rAU)*fvc::ddtCorr(U,phi)
);

I do not understand what the operator "ddtCorr(U,phi)" means and what is the expression or equation corresponding to this operator?