d-CON works by inducing internal bleeding. The active ingredient in d-CON bait traps is a chemical called "brodifacoum." It prevents blood from clotting and also damages capillaries.
When a pest eats the bait in the trap, the poison is absorbed into its blood through the gastrointestinal tract, causing damage to small blood vessels of the stomach and making it impossible for blood to stop flowing out through the wounds. Normally, clotting would slow and stop the flow, but brodifacoum's anti-coagulant properties prevent this natural protective measure from happening. Eventually, the pest will collapse from loss of blood. The blood loss continues until the pest dies.
Brodifacoum is not a fast-acting poison and usually takes four to five days to kill following the first ingestion. Because it causes the animal no obvious pain or distress at first, the pest will not develop an aversion to the poison and continue to feed from toxic bait. Food aversion is a problem with more fast-acting poisons, as the pest will quickly notice that something is making it ill and avoid eating a fatal dose of the bait. Aversion to bait can spread throughout the entire pest population. This makes slow-acting poisons more effective at destroying pests overall despite taking longer to kill.