In a Diels-Alder reaction, what is the endo rule?

When the dienophile has a pi bond in its electron-withdrawing group (as in a carbonyl group or a cyano group), the p orbitals in that electron-withdrawing group approach one of the central carbon atoms (C2 or C3) of the diene. This proximity results in secondary overlap: an overlap of the p orbitals of the electron-withdrawing group with the p orbitals of C2 and C3 of the diene. Secondary overlap helps to stabilize the transition state. The unsaturated substituent on the dienophile generally prefers the endo position in the transition state, giving rise to the endo product in bicyclic systems.