Tren basically causes a systemic allergic reaction in the body, causing constriction of the airways(bronchioles) and also the blood vessels in the lungs. This severely reduces the bodies ability to take in air and oxygenate the blood, thereby reducing the bodies aerobic endurance capacity. This seems to be for most people, but not all.
Nasacort fights multiple types of chemicals in the body that the body releases in response to exposure to allergens. Most people are familiar with histamines, if you have seasonal allergies you certainly are. But the body also produces other chemicals in response to allergens. Leukotrienes, prostaglandins, eosinophils and cytokines are produced as well as histamines, depending on the nature of the allergen and the level of exposure. I think it's mostly prostaglandins that cause the pulmonary constriction associated with the use of Tren. Nasacort blocks the release of the prostaglandins and relieves the swelling, which allows the body to once again exchange oxygen properly, and increase cardiovascular endurance.
Flonase would also work, I just prefer Nasacort because it has no alcohol, no smell or taste, and it was the only one available when I first figured this all out.