How to Stitch Through the Front Loop Only (flo) or the Back Loop Only (blo)
Working into just one loop in amigurumi is a fantastic trick for shaping the edges of your pieces, or for changing the direction you've been crocheting in. You'll usually see it written as flo (front loop only) and blo (back loop only), and it's a go-to technique for shaping all sorts of pieces that call for crisper, sharper edges.
Single Crochet Back Loop Only (sc blo)
Step 1. Push your hook into the back loop only of the next stitch. Yarn over and draw up one loop. You should now have 2 loops sitting on your hook.

Step 2. Yarn over again and pull through both loops, exactly the way you would for any standard stitch.

Step 3. To really see the effect take shape, work a few of these in a row and watch how the form starts to flatten, almost like a sharp fold.

And there it is — your first single crochet back-loop-only! This one comes in especially handy for amigurumi that need to stand flat, or for crocheting elements with straight, clean edges.
Single Crochet Front Loop Only (sc flo)
Step 1. Push your hook into the front loop only of the next stitch. You should now have 2 loops on your hook.

Step 2. Yarn over and pull through both loops, just as you would for any regular stitch.

And that's it — your first single crochet front-loop-only is done!
Step 3. To best see the effect, work a few in a row and notice how the shape opens up and out toward the exterior, much like the brim of a hat.

It can be tricky at first to know whether a spot calls for a blo or flo stitch, but the more you practice, the more natural it becomes — and it makes a real difference in the range of shapes you're able to build.