Oil (or ghee) is heated until extremely hot, but not burning. Then spices, generally whole, or chopped-up garlic and ginger, are added to it. These whole spices straight away swell, brown, pop or otherwise change character.
This seasoned oil, with the spices, is then poured over cooked pulses and vegetables, or else uncooked foods are added to it and sautéed. Those seasonings most commonly used include cumin seeds, black mustard seeds, fennel seeds, whole dried red chillies, cloves, cinnamon sticks, cardamom pods, bay leaves and black peppercorns, as well as chopped-up garlic and ginger. Hot oil transforms them all and gives them a new, more concentrated character.