We didn't go into this in the news post since it was a quick overview, but GIMP uses GEGL for its main color and pixel engine. GEGL has a feature where you can automatically sync data between "buffers" in memory and in file - so as you're editing the image/layer/etc in GIMP, the buffers in the new format will automatically auto-save the data from the canvas.
It's still in-progress, so we'll have more technical details in the 3.3.2 release news.
It's still in-progress, so we'll have more technical details in the 3.3.2 release news.