Part 1 demonstrated how a full step takes place and there are 4 steps per revolution. If we were to go to a half step sequence we could obtain twice as many steps per revolution of 45 degree steps.
What we do is to sometimes allow both loops of wire to be on together.
One thing to note is that when two coils in close proximity are energized there is a resultant magnetic field whose center will depend on the relative strengths of the two magnetic fields.
Fig 27A demonstrates this.
The sequence for half stepping works like this as in fig 27.
45 degrees is still a very crude step. How do actual motor achieve movements as low as 1.7 degrees. etc.? The next section Basic Stepper Motor Theory Part 3 (Practical motors) covers this material.
You may like to revisit the previous section Basic Stepper Motor Theory Part 1 (Full Step)