In Diels-Alder reactions, the interaction occurs between the Highest Occupied Molecular Orbital (HOMO) of the diene and the Lowest Unoccupied Molecular Orbital (LUMO) of the dienophile. This is crucial because the diene donates electrons from its HOMO, while the dienophile accepts them through its LUMO. The breaking of the bond in the dienophile during the reaction necessitates that its LUMO is populated with electrons, allowing for the formation of new bonds. If the HOMO were to depopulate instead, it would result in no electrons remaining on the dienophile, preventing the formation of the desired cycloadduct. Understanding this electron donation and acceptance mechanism is essential for predicting the outcomes of cycloadditions.