Final answer:
During CPR, a single rescuer is not recommended to use a bag-valve mask (BVM) because it demands the use of both hands, making it difficult to perform effective chest compressions simultaneously.
Step-by-step explanation:
The ventilation device/technique that is not recommended for a single rescuer to provide breaths during CPR is the use of a bag-valve mask (BVM). During CPR, a single rescuer may not be able to effectively use a BVM as it requires both hands to maintain a proper seal on the mask and to squeeze the bag, which leaves no hands free for chest compressions. The proper positioning of the hands on the sternum is crucial for effective chest compressions, which is a key element in maintaining blood circulation when the heart stops. CPR courses and certification teach rescuers how to prioritize tasks and use the most appropriate methods during a resuscitation attempt.