If anyone is interested: within the cells of the garlic clove, the two precursor molecules of allicin (alliin and an enzyme called alliinase) are kept separate. Alliin is stored within the cytosol of the cell, whereas the alliinase enzyme is sequestered in bubbles called vacuoles. When the cell wall and vacuole is disrupted through crushing or chopping (or attack by insects, fungi etc) the two molecules meet and react, eventually forming the short lived molecule allicin, which as has been pointed out, contributes significantly to garlic's characteristic aroma and flavour.
Incidentally, the compartmentalisation of the allicin precursors protects the garlic plant from self harm. As the allicin molecule is highly reactive, collateral damage to the plant's own cells is localised only to sites of tissue damage from microbes and other attackers.