The embouchure remains intact throughout this quick and simple movement. It is important to 'suck' air to the lowest regions of the lungs. The lungs are pear shaped and need to be used in their entirety hence we should not inhale by raising the shoulders. Nor is it necessary to over-fill or take in any more air than is necessary to play the following note or phrase. The oxygenation process continues while inhaling through the mouth and exhaling through the trumpet.
Teachers speak of having good 'breath support'.
What they actually mean is the power and speed at which the air can be blown out through the trumpet. It is very similar to the action used when 'huffing' on a cold window- pane. This expiratory movement is controlled by the abdominal muscles forcing the diaphragm upwards thus causing air to be forced out. The circular iris type aperture in the lip causes some resistance to the flow of air. A low note needs a larger aperture and therefore causes less resistance.