Click the structures and reaction arrows in sequence to view the 3D models and animations respectively
In this example, the carbonyl group is conjugated with the dienophile. The less stable cis ring junction is formed because the molecule can fold, so the carbonyl can enjoy a bonding overlap with the back of the diene.
A word of warning: intramolcular Diels-Alder reactions may give the endo or the exo product. In this case, the endo product is obtained, however, you should be prepared for either the exo or endo products, or a mixture of both!