In modern society, opportunities to express ourselves physically and emotionally are not always available to us. Offices, for example, are often very polite and restrained environments, and it may not be possible to express all the frustration or joy that a day's work therein inspires. These emotions don't always simply fade away when suppressed. Sometimes other opportunities need to be found to let out this emotional energy.
An outlet for this emotional is called catharsis, and can be seen in gymnasiums and on jogging trails where people use their physical exercise not just as a means to get or stay fit, but also to release this excess energy.
Catharsis can also occur in less physical activities, such as meditation, or quiet strolls in the countryside, but the more intense the activity or experience, the quicker the release.
Some BDSM activities provide the physicality which lends itself to cathartic release. Flogging (both giving and taking) is a good example of this, as are piercing, nipple torture, and other pain-based activities.
In terms of BDSM, this process of catharsis occurs when an intense BDSM experience creates a context which allows the release of nervous or emotional energy.
When looking at flogging as catharsis, heavy flogging (where the experience is more of a thud than a sting) is more likely to provide an outlet.
Signs that there is nervous or emotional energy needing release can be irritation or restlessness.