It's essentially leveraging & managing the natural succession of soil to forest. It uses support species to produce bio mass in situ, provide shade/shelter for short, medium & long term crops.

Systems are reset lightly or heavily depending on the primary crops you're focusing on & what you're wanting. This provides bio mass above ground but also collectively stimulates all plants in the system via the roots.

https://www.youtube.com/watch?v=gSPNRu4ZPvE
 

 Caveat - I'm still a newbie & don't know what I don't know 

 That's one of the beautiful things about working with nature. It is so clever that it's humbling.  We will never know quite how it works.