Band-resisted ab rollouts are usually done facing the band straight-on so the resistance is primarily front-back. While that's fine, setting up at a 45-degree angle to the band but still performing them just as you normally would (moving straight back and forth) creates a unique challenge.
You're still moving purely in the sagittal plane, but you now have to resist getting pulled sideways by the band too, which forces you to stabilize in the frontal and transverse planes to resist rotation and lateral flexion, thereby working the rotary and lateral core along with the anterior core.
I call them "anti" rollouts because they train anti-extension, anti-rotation, and anti-lateral flexion all at the same time.
These are a lot harder than they look. The band will have a tendency to pull you towards the anchor point, especially as you reach the point of full extension, so it's important that you're sure to move out and back in a straight line.
You can easily increase or decrease the difficulty simply by setting up farther away or closer to the anchor point of the band. The farther back or the farther off to the side, the harder it is.