When I tuck in a sweater, I make sure the sweater is lined up correctly with the center of my skirt, jeans, or pants.

For this look that got all the questions, my sweater was only tucked from center to where the pockets start. The rest of the sweater was untucked and just bloused under.

Here, my sweater had slits and I only tucked in the front. By doing this, it ended up being tucked in beyond the slits, which helped the back part blouse under on its own.

Most of us curve in at the small of our back and curve out on the opposite side. By having the bit of bulk in the back of your look, it balances everything nicely.

I kept the sweater completely smooth in front of the skirt, and let the gathering happen at the small of the back. I bloused it out a bit to make space for the gathering. 

You can see how I pulled the side volume to the front and it almost created an architectural detail. It created more of a waist shape.

I only tucked in a couple of inches of this thick sweater. Any more, and the drape would be lost and with it my shape and my comfort.

This sweater was fully tucked but because it's lightweight it didn't add bulk. By gathering the bulk at the small of the back, it still achieved a high-low effect.