Invisible Decrease

Find the front loop (in blue in the picture) and the back loop (in yellow in the picture) of the next two stitches.

We'll only work the front loops (blue).

Insert your hook into the front loop of the next stitch.

Then, keeping the front loop of the first stitch on the hook, insert the hook into the front loop of the second stitch.

Yarn over and pass it under the 2 front loops on your hook but not under the  loops on the hook (red cross).

Yarn over again and pass it under the 2 loops of your hook.