How can reducing preload and afterload help the heart in regards to heart failure?

Heart failure: Preload reduction lowers the filling pressure of the heart and the back pressure in the lungs causing shortness of breath. Afterload reduction lowers the resistance to ejecting blood and makes for better forward blood flow from the weakened heart. We don't generally use afterload reduction for diastolic heart failure unless hypertension is present.

Your heart : Will relax in the case of systolic heart failure improving symptoms. In case of dyastolic heart failure that management will be contraindicated or use with a lot of caution.