There are flowers growing around a heart with a wick sticking out of it. The heart is a sacrifice to the old gods and the life energy bleeds from the burning wick to feed the flowers. The wick burns, releasing the power of the heart which spreads into the ground around the heart causing the flowers to grow.
Sometimes the wick burning the heart isn’t enough for the flowers to grow properly and they get thirsty for the blood from your shin (しん). So, you slice open your shin with a sacrificial knife and drip your blood over the thirsty demon flowers.