Just as an increase is used to start making your amigurumi shapes larger, a decrease is used to start shaping it down again. I prefer an invisible decrease to a regular decrease, just because it’s (as the name implies) harder to spot, and therefore neater – and I also happen to find it easier to complete!
We’ll be joining two stitches. Insert your hook into the front loop only of the next stitch.
Then, keeping both loops on the hook, insert your hook into the front loop of the second stitch.
There will be three (probably tight) loops on the hook, yarn over and draw through the first two loops.
You will still have two loops on your hook. Yarn over again, and draw through the remaining loops.
Done! Going through the front loop instead of both loops is what makes it “invisible”, and gives it a neater, closer decrease.