Bag-mask ventilation is a technique used to provide positive pressure ventilation to a patient who is not breathing adequately. It involves using a self-expanding bag connected to a facial mask that covers the patient's mouth and nose. The rescuer squeezes the bag to deliver air into the lungs, ensuring that the mask forms a proper seal to prevent air leaks. This method is commonly used in emergency situations, such as during cardiac arrest or respiratory failure. Proper technique is essential for effective ventilation, which includes maintaining an open airway and ensuring the mask fits snugly on the patient's face.