This formula relates to how Pauling determined the values for electronegativity for all the elements and how delta (if big) shows that the molecule is very polar. Delta being:
$\Delta =\Delta H\left(AB\right)-\Delta \:0.5\left(\Delta \:H\left(A2\right)+\Delta \:H\left(B2\right)\right)$
$\Delta H\left(AB\right)$ is the experimental bond dissociation energy. The difference in experimental and theoretical bond dissociation energy is caused by the electronegativity of the molecule.
The more electronegative the atom the bigger the delta value. But why is that?