Basically, you're working your ab muscles without doing a single sit up or crunch. Pretty awesome.
For stability during pregnancy, I planted my feet on the ground and did the twist move for 30seconds per the Kettlebell360 app instructions. However, there are a few variations on this move to make it more challenging.
As the video shows, crossing your feet and raising your heels up off the floor will make for a more challenging move.
You can also lean back more to engage your abdominal muscles, and straighten your legs out a little further in front of you.
It's a great exercise, especially when you are twisting with weight. In the video, a 30lb weight is used. If you're pregnant and just starting out, go lighter. I used a 10lb kettlebell throughout my pregnancy and that was definitely enough weight for me!