[cmgxyz]

The short answer: very extensive systems of phonetic shorthand, a special keyboard with very few keys designed for chorded input (multiple keys pressed simultaneously), and specialised software dictionaries to translate the shorthand back into English. Professional stenographers can type upward of 300wpm as standard. The downside is that it takes years to become proficient. http://www.openstenoproject.org/ is an interesting resource if you're curious.