Expanding memory of Atari 400 (Piggy Backing)

I would like to learn more about piggy backing memory.   When I got my first Atari computer we had enough money to buy a Atari 400 and my dad used his experience working with the home built Sol Terminal 20 to piggy back the memory and boost the ram card from 16k to 48K.   Worked great.

How exactly does this process work when you are piggy backing ram?   What needs to happen to make it work?

And if you don't know, but have photo examples you have found that you want to show off, Im would find that fascinating as well.

I found this article for piggy backing on a Z-80:

http://www.classic-computers.org.nz/system-80/hardware_mods_memory_upgrade.htm

And the attached pictures were a piggy backed board I found in a collection.  I plan on adding pictures of my dad's work soon.