Narcissists are embarrassing creatures; they do things that are utterly embarrassing to satisfy their addiction. They will literally say and do the most embarrassing things just to achieve the gaining of supply.
To gain narcissistic supply, the narcissist sometimes has to behave in a manner that suggests a low iq. For example, repetition is a key manipulation tactic and so the narcissist tends to repeat themselves over and over trying to drive the victim insane.
Another example of how a narcissist embarrasses oneself is by acting offended by an action or inaction of their victim such as when the victim asks the narcissist to pass the salt and the dinner table . “You can get the salt, do you really think you can control me?”
The last example I will give you is when the narcissist makes up lies and stories when talking to the victim. “I know you cheated so why are you lying?” Meanwhile, the victim has only been to work and back for the last three months like clockwork.