This is simply using the words "fuck" and "shit" in one's first language, not gratuitously but when the impulse arises. At first it may be necessary to allow the suppressed impulse to arise. Other words may suggest themselves, what is profane varies slightly from community to community. For instance, "ass" may or may not be considered profane. In sex, referring to what is happening with the simple words that suggest themselves is known as 'talking dirty' and usually enhances arousal for the reasons mentioned in the rationale.
Profanity gets intangled with two other issues, misogyny and swearing. Misogyny is using the sexual aspects of a woman to demean or dehumanize her. This is a matter of attitude not word choice. Swearing is using the name of a diety or of the sacred "in vain" or characterized by 'wordly' motives. Neither misogyny or swearing is necessary in the use of profanity. To call sexual matters 'sacred' just to mandate euphemism and indirectness is a misuse of the term sacred.
Although the biological benefits of profanity are easy to demonstrate with self-experiment, social mores are strongly against it, and punishment may ensue. Social context is relevant. However, the compellingness of profanity gives rise to a split in practice. For instance, children hear swearing frequently 'accidentally' but should a speaker seem to do it deliberately, it is a case for censure. To insist children wait until 18 to use profanity makes no biological sense.
Rationale: Basic profanity connects the limbic areas of the brain with cortical areas. This connection is often tenuous. Where profanity is non-stop and compulsive, the connection is weak and the limbic area tends to intrude in behavior 'ungoverned' Where profanity is absent or strongly inhibited, the connection is weak and behavior is emotionless. What is desireable is a balance. Trying to avoid profanity or the most basic words for anatomy in sexual activity is an act of inhibition that inhibits arousal overall. There is a link between the suppression of profanity and the supression of sex. Profanity can constitute a discharge that helps incrementally to limit kindling.
There will not doubt be many who find this practice unpalatable or wrong nonetheless. It is of course inessential. The intended benefit, as with all these practices, is to increase harmony within the person and if this practice does not seem natural fairly quickly, then that goal is not achieved.
Source: Folkwisdom although this has been studied for instance see: Stephens, R. & Umland, C. (2011). Swearing as a response to pain – effect of daily swearing frequency. Journal of Pain, 12, 1274-1281. doi:10.1016/j.jpain.2011.09.004