The invisible increase technique in crochet is a method used to add stitches to your project without creating noticeable gaps or bumps that can sometimes occur with traditional increase methods. It's often used in amigurumi and other projects where a seamless appearance is desired.
Insert your crochet hook into the back loop only of the stitch where you need to make your increase (instead of under both loops as you normally would).
Yarn over and pull up a loop.
Look after step 2
You should now have two loops on your hook.
Yarn over again and pull through both loops on your hook, completing a single crochet in the front loop only.
Look after step 3
Insert your hook back into the same stitch, this time going under both loops as you would for a regular single crochet.
Yarn over and pull through the stitch.
Look after step 5
Yarn over again and pull through both loops on the hook.
The invisible increase is now complete.