Before I do that, I want to have a very solid understanding about what the risks and benefits are and where the 20% interest comes from.

Well buddy, and since you ask it in such a sweet way and with an investor mindset plus with the intention of attracting new investors. Then, to start, we're going to provide you with a big bag of laundry soap to keep you busy for a good while. ;o)

You may start by washing these baskets of laundry first:

1. @deathwing/decreasing-hbd-apr-and-posting-rewards-and-increasing-the-importance-of-hive-power-hp

2. @gtg/gtg-witness-update-upcoming-changes-in-hbd-apr

3. @alokkumar121/expect-apr-reduction-on-hbd-in-savings

4. @taskmaster4450/the-apr-on-hbd-savings-should-not-be-lowered

5. @edicted/hive-has-officially-reached-the-desperation-phase

6. @anomadsoul/dissecting-tokenomics-and-game-theory

Can anyone point me to any thorough explanation about how HBD works and where the 20% interest comes from?

Well, and to answer these two questions. If by chance in the links already provided you do not get a satisfactory answer to these last questions. You always can try go and snoop for a while through the tag #HBD and see if you can find something useful that can complement the information already provided.