You said starting so I assumed there would be a progression. Overall it doesn't seem like a good idea to me because it puts the whole weight over the lower back without any back and arm support (like the ones that exist on a good office chair, they're recommended for a reason). Also, there are a ton of good exercises for the core that don't stress the back, like planks for example.

Anyway, I recommend that people ask their doctor for advice or at least Google up a bit about the pros/cons of this and try to find articles from reputable sources. I did that some time ago and wasn't very impressed with the results.