Die Hard improves the foundations for an erection, increases sperm production and motility and increases the testosterone level.
For the swelling of the 'Corpus cavernosum' (the swelling of the penis) you need an 'endothelium-derived relaxing factor'. To bring about the endothelial factor you need the presence of sufficient nitric oxide. Nitric oxide is formed from L-Arginine (with the help of free intercellular Calcium) and ensures the relaxation of the Corpus cavernosum and as such an erection. Age, bad habits, etc, reduce the presence of sufficient Arginine. Die Hard complements this presence. It is not a drug, like Viagra, but a supplement.