Final answer:
The next step in the 1-2-3 Breathe drill progression after mastering the side glide with kick is to add arm movements while synchronizing breathing with the swimming movements.
Step-by-step explanation:
After mastering the side glide with kick, the next step in the 1-2-3 Breathe drill progression for the butterfly stroke is to introduce arm movements. In the butterfly stroke, the swimmer must synchronize their breathing with their swimming movements. This is because breathing, or ventilation, is crucial for maintaining proper rhythm and efficiency in the water. Ventilation is the process of moving air into and out of the lungs, which is vital as the lungs perform gas exchange during the respiration process. To progress in the 1-2-3 Breathe drill, the swimmer should focus on integrating the breathing technique with the undulating body motion and simultaneous arm movements that define the butterfly stroke.