  Enhancement, due to constructive interference through the many possible
reaction channels, occurs in two-neutron transfer reactions from a bound system
$A$ to a weakly-bound system ($A$+2), whenever the intermediate system is
unbound and consists of a continuum with a resonant state. We elucidate this
phenomenon in the case of $^6$He, modeled as two neutrons in the orbitals of an
intermediate $^5$He nucleus, comparing the realistic case of an unbound ($A$+1)
system with an artificially bound case. A properly modeled continuum in these
calculations is found to be crucial and leads to significant enhancement due to
pairing correlations.
