What is "self-active" play?

I recently read the book, "From Play to Practice" from NAEYC.  Near the beginning of the book, it talks about "self-active" play.  This is a term I had never heard before, but once reading about it, it seems pretty simple:  "self-active" play refers to spontaneous play in both children and adults.  This spontaneous play is in conjunction with the use of open-ended materials to support the play.  "The environment, the materials, and the children's natural curosity combine to activiate the brain to construct new meaning."  

I had often considered this type of play "open-ended"; we intentionally present the open-ended materials in hopes that it will spark the children's sense of curosity and initiate exploration and discovery.  

One of my personal discoveries is when we can set out a "provocation" with certain intentions or learning goals, and how the play always surprises us and takes us somewhere we had not considered.  That is the beauty of the Kindergarten program.  It gives us the freedom to look back and reflect upon the learning and name the learning.  I am always surprised just how many curriculum goals we achieve; so much more than or original leanring goals and always so much deeper and richer!