"The typebars hung in a circle. The roller which held the paper sat over this circle, and when a key was pressed, a typebar would swing up to hit the paper from underneath. If two typebars were near each other in the circle, they would tend to clash into each other when typed in succession. So, Sholes figured he had to take the most common letter pairs such as "TH" and make sure their typebars hung at safe distances. "
A bit more articulately stated, but the very same idea: Forcing the typist to hit keys that were attached to arms not immediately adjacent to each other. I actually typed for a living (military clerk) in the the very early seventies using the old style typewriter. If you really rocked and rolled you could get the key "arms" to jam.