Increase via the backward loop

Make a loop of yarn as shown on your left thumb.

Insert the right needle into the loop and remove the loop from your thumb.

Gently snug the stitch and knit the next stitch as directed.

Simple Double Central Increase
The following all happens in the same stitch.

Knit the stitch through the back loop (ktbl), do not remove stitch from left needle.

Work increase via backward loop (as in tutorial above).

Knit stitch through the back loop again and remove from left needle.

Your increase should look like this.

I have used this increase in a few different situations that called for the standard central double increase. I find this method to be less fiddly (ie: not as tight) and still provide good results. You are welcome to use which ever method you like best.