I practice that myself. Maybe you're turning the double play and you want to stay down because the ball is thrown low, so maybe you throw sidearm. Maybe the infielder giving a feed at second base might throw sidearm. I definitely wouldn't suggest doing it. In the long run, it will be hard on your arm.
Some guys actually leave their feet on their follow-through.
Make the throw out in front of you so you get backspin on the ball. It's more important that you're under control and the throwing motion is a smooth motion for your entire body. Catch the ball on the throwing-arm side of your body. That way you can make the transition of the ball from your glove to your hand more quickly. It saves time and cuts down on movement.