Intracardial injections, commonly know as heart sticks, is a rather inhumane way to euthanize an animal that is not (or nearly comatose) or heavily sedated. A needle and syringe containing is passed through the chest wall and severely layers of muscle into the heart. However on an animal that isn't sedated or comatose this is hard to do because the body is in constant motion. It's not uncommon for the person administering the injection to miss and accidentally punctures the lungs and causes them to fill up with fluid. This is a VERY painful way to die.
In a shelter there is no reason for this to be done on an otherwise healthy dog. In a clinic however, it is often needed for dogs that need to be euthanized but have circulatory problems.