In general, extension packages are a set of living books that are added to HOD core programs in order to extend the learning for older dc in the area of history. Target age ranges indicate the level a core HOD guide is written for, and extensions extend that learning, so older siblings may learn along with younger siblings, and be combined in the same core guide. You can look at the HOD catalog or HOD placement chart to see the span of target ages, and the span of extension package ages for each guide. Extension packages first become available in Bigger Hearts for His Glory, and then continue to be available every year after. Bigger Hearts for His Glory is the only guide that includes science and history titles within the extension package - the rest of the upper guides only include history. All extension packages match the flow of chronological history. They have a schedule with pages to be read independently by the student in each HOD guide's Appendix. Follow-up activities are suggested in each guide's Appendix, and a student is to rotate through those follow-ups each week along with the scheduled reading. Extension packages are more mature in content and are intended for the older age levels of dc indicated by the placement chart. They are intended to be read independently by the student, and were chosen for this purpose. Thus, if a parent chooses to read them aloud (at the upper levels especially), the readings will feel long. Of course a person can do this, and may enjoy doing this, but the purpose the extension packages were written for was to help older students extend their learning by reading titles appropriate for their age level independently.
How does it extend their learning? In many ways. First and most obvious, it extends their learning in the area of history. A deeper look at history with a wider view of what it was like to live during that time period is gained by reading these books. Second, it extends their learning in reading, as they are reading more books each day. Third, it extends their learning in LA Charlotte Mason type skills, as their follow-ups include in their rotation oral narrations and written narrations, of which the length and the specifics of increase each year. Fourth, it extends their learning in the designing follow-up higher level thinking activities that are more open-ended and draw upon students' creativity, as well as their ability to pull what was learned all together and show what they learned in a final response that meshes their core program learning with their extensions. So, notebooking type responses with specific guidelines are used, for example. Fifth, it extends their learning by requiring a more mature level of independence. They are responsible for following their reading schedule of extensions and completing and turning in their follow-up assignments (this is an excellent stepping stone for secondary style education).
This past post may be beneficial to read. In it, Carrie addresses how to get the most out of BHFHG's extension package for dc:
viewtopic.php?f=6&t=6095&p=44665#p44665
I hope that helps!

The extension packages offer an amazing tool to extend HOD's core guides for older students without adding much more to a busy homeschooling parent's day. Save for correcting assignments and giving feedback on them, a student should be learning independently within the extensions, which is an important skill to learn as dc mature.
In Christ,
Julie