Why crushing the clove matters
Intact garlic stores a stable precursor called alliin in one compartment and the enzyme alliinase in another. Crushing or chopping ruptures the cells and lets the enzyme convert alliin into allicin, the pungent, short-lived sulfur compound. Allicin itself is unstable and breaks down quickly into further sulfur species, so the timing of crushing and cooking affects how much active material survives. Letting crushed garlic stand for around ten minutes before heating is often suggested to let the reaction complete.
Nitric oxide, platelets and blood pressure
Garlic-derived sulfur compounds are linked to increased availability of nitric oxide, the signalling molecule that relaxes the smooth muscle in vessel walls, which can produce a small lowering of blood pressure. Separately, these compounds appear to reduce platelet aggregation, the clumping step in clot formation. Meta-analyses of garlic supplements report a modest average reduction in blood pressure, especially in people who start out hypertensive, though effect sizes vary by preparation.
Because the antiplatelet effect can add to that of blood thinners, anyone on anticoagulant medication or facing surgery should discuss garlic supplements with a clinician. This is general educational information, not medical advice.