IFPDOn[exp_, q1, q2, ...] changes from FeynAmpDenominator[...] representation to the IFPD one (Inverse Feynman Propagator Denominator). I.e., FeynAmpDenominator[PropagatorDenominator[a,b]] is replaced by 1/IFPD[a,b] and the q1, q2, ... are the integration momenta.