Well, normally when you make a slip stitch to close a round, you have one loop on the hook, you put the hook in the gap where the sl st will go, and pull the yarn through all loops on the hook to make the sl st. This leaves that first loop in *front* of the fabric.

All I'm doing in steps 3 and 4 is making sure that the loop ends up *behind* the fabric, so, if you've managed to get the loop back onto your hook in step 3, all you need to do to get the sl st in step 4 is pull a loop of the new color of yarn through all loops on the hook.