  Taking the sum rule for the rho meson as an example, we study the possibility
of explaining the phenomenological enhamcement factors for certain terms in the
vacuum expectation value of the operator product expansion in the QCD sum rule.
We take a QCD motivated extended Nambu-Jona-Lasinio (ENJL) model as the low
energy effective Lagrangian for QCD with which we calculate the nonperturbative
contributions to the vacuum condensate expansion to obtain the enhancement
factors. Our result shows that such nonperturbative contributions can cause
large enough enhancement factors which can be consistent with the
phenomenological values.
