The reason this is powerful is because the last part becomes the most practiced and the learner is always moving towards the more familiar and well-rehearsed behavior. So the forward movement is reinforcing in itself.

A good example is learning a piece of music (but the same goes for any type of chained behavior).

Usually people start at the beginning every time, adding more to the end as they become familiar with the first part.

When it comes time to perform they are moving more and more towards the part that is the least well-rehearsed. They are anxious the whole time because the part with the most chance of an error is coming up.

If they started by learning the last few bars, then the ones before that and so on, they would always be moving toward the most rehearsed part.

At the performance they would gain confidence all the way though, rather than losing confidence and becoming more likely to make a mistake.