In bars 5-6 the lower note jumps an octave. Why is this good?
To me it sounds like it is not musical enough. Too big of a leap. Please help me understand this octave leap.
I have seen octave leaps in hymn eg both tenor and bass being on sol with the bass jumping down to sol an octave lower but that sounded very good.
In this style, an octave leap (almost always ascending) is acceptable as long as you resolve the leap in the opposite motion by step. In other words, an ascending octave must then move immediately down by step.
In fact, this is true of most large leaps in this style. If you look at both voices of this example, you'll notice that almost every leap of a third or larger resolves with a step in the opposite direction. The only exception is the descending third in m. 4 of the lower voice; thirds don't always resolve by opposing step, but they often do.
An example of the principle that Richard describes:
