In my opinion this is not a very common crochet stitch but I like to use it when I really don’t want a sharp or deep drop between a double crochet and a treble crochet. I use this stitch in my crochet pumpkin leaf tutorial and here is a tutorial on how to make this stitch:
You will yarn over (yo) twice on your hook (just as if making a treble stitch) ~~ then insert your hook into the stitch or space according to your pattern directions ~~ now yarn over (yo) again and pull through that loop (at this point you will have 4 loops on your hook ~~ yarn over (yo) and pull through the first 2 loops on hook ~~ yarn over (yo) again and pull through the last 3 loops on hook. I’ve included some pictures too:
It just makes a subtle difference in the look of your leaf or whatever you are crocheting.