You can see the (inner) shifting mechanics of a Shimano Nexus8 gear hub in action.
The first gear is driven directly using the sun wheel, the ratches will then activate the inner epicyclic gears (gear 2-4). Gear 5 is driven by the then activated clutch on the gear cage, after that the ratches will activate gear 6-8.
This hub has run 14.000 km so far and was serviced only once before.