How we change what others think, feel, believe and do
Language and words are how we encapsulate meaning. Hence, if you control language, you control thought.
Words are little capsules of meaning. They are symbols upon which we hang bagsful of inferences and understanding. We think in words and sentences, and without language (and sign language follows rules of grammar as much as verbal language) our ability to understand and think would be far, far less.
New words for new meanings
When something new is discovered, then we give it a new word. This separates the new thing from other things. Having a separate word makes it a separate thing, with different meaning.
Philosophers and scientists create new words to indicate that they have discovered something new. This effect is also used by those who want to give the impression of having discovered new knowledge. If I talk about the universarium of time, you might be forgiven for thinking I have discovered something new and valuable.
Old words for new meanings
The reverse can also be done, in that existing words can be redefined to have different meanings. Teenagers and advertisers regularly do this, and superlatives from many different domains have been pressed into new service. 'Fabulous' means 'like a fable'. 'Fantastic' means 'like a fantasy'. And so on.
Using old words for new meanings creates exclusivity for those 'in the know' and allows them to talk openly amongst those who do not know and appear either to be talking normally or talking nonsense. Either way, it permits in-group conversation that not likely to threaten other people. It also creates a duper's delight at having a 'secret language'.
Using special words and language can lead to significant influence and control of other people.
Words contain and trigger emotions. Think about swearing, children, crime, movie icons and more. With a few choice words, it is possible to evoke most emotions in other people. Power words are a typical example of this.
Controlling words thus also controls emotion. The use of evocative words alone is enough to change people. And when you have created the meaning of the words yourself, including the emotional content, then you have even further control.
The allure of special words
Groups and leaders often keep special words for use only within inner circles of power. These then become symbols within the inner group of its exclusivity and also become attractors to others who want to join the inner group and learn these special words.
The meaning of the special words may be revealed in careful illuminating rituals, where people who are being elevated within the order are given the power of understanding this new language.