BLW readiness question

I know the child should be “sitting independently” before starting, but what exactly does that mean? Should they also be showing signs of protective reactions (arms to the side as they fall over? My boy isn’t quite doing that yet, but has excellent head and trunk control when in his high chair. He still topples over when we practice sitting. Any tips on helping him learn? He is very interested in food and gets frustrated not being able to try it! 😆