Because swearing is cool and I'm a 12 year old stuck in a middle-aged body, this has its own help section. Swearing is disabled by default so once you've got Crew Chief working you'll want to enable it because swearing is cool. Look for a checkbox in the Properties screen called Use sweary messages (search for sweary using the search box) and tick it. Crew Chief will swear where appropriate - it's not like listening to an Eddie Murphy gig and doesn't happen very often but it does help add a bit of personality to the messages.

Kind of related is Enable pearls of wisdom which is normally ticked and means that Crew Chief will give you both positive,  negative and neutral "general encouragement messages"  from time to time (adjustable with Frequency of pearls of wisdom)