Exert

A weightlifter is trying to lift a heavy barbell. His muscles tense, his face strains, and he pushes with all his strength. He may or may not succeed, but he is definitely applying maximum physical effort. What is the weightlifter doing? He is exerting force.

Occasion (to create a situation where something can happen, often indirectly) contrasts with exert, which means to apply active pressure, force, or effort to achieve an effect, regardless of the outcome. Exert emphasizes the application of energy, while occasion emphasizes providing an opportunity or context.