The 3 speed internal hub is mated to a multi speed cassette instead of a single driver, the latest version is a 9 speed that gives 27 speeds off the rear.

It is like having a derailleur gear system with a low, medium, and high range.... some lunatics mate these with double and triple cranks to get a possible 81 speeds.

My 7 by 3 dual drive gives the same gearing as a touring bicycle with a single crank, I have a 9 by 3 that I will be installing soon and this is just to give closer steps on the cassette.