The POWERbreathe device will try to maintain a variable load for each breath, if you fail to overcome the load set during a course of a breath most likely due to fatigue of the respiratory muscles you will see a sudden drop of pressure plotted. In essence you have reached the point of failure, where the muscles have reached a point where it cannot overcome the load and fatigue has taken place. This is commonly seen at the end of a training session, if you see this behaviour early in the training session then it is a good indicator that the training load set is too high or you need to put more effort in each breath to maximise your volume.