Final answer:
In the head tilt chin lift method, you should not allow your fingers to press deeply under the chin to avoid obstructing the airway or causing injury. The fingertips should be used gently to lift the chin and ensure the airway is clear.
Step-by-step explanation:
When performing the head tilt chin lift method of opening the casualty's airway, you should not allow your fingers to press deeply in the soft tissues under the chin. This method is used in first aid to clear the airway of an unconscious person when there is no suspected spinal injury. To perform it correctly, one hand is placed on the forehead to tilt the head back while the fingers of the other hand are placed under the point of the chin to lift it. The pads of the fingers should be used to lift the chin, not the fingertips, and care must be taken to avoid pressing too hard on the soft tissues under the chin which could obstruct the airway or cause injury.