The ability to do this comes from the experience of knowing when you've passed the point of no return with your brace, and a roll is imminent. The quicker you realize when you've reached this point, the quicker you can start your roll. Understanding that there is no such thing. A roll is a roll is a roll. Lake, river, waterfall-the technique and execution never change.
People panic for no reason.
Remember, it's better to be in you boat, mostly out of control, than out of your boat, completely out of control, with no chance of ever getting it back. Totally, unequivocally, believing it's gonna work. Most students are surprised to find that the roll they have mastered in the pool doesn't work when they capsize on the river. There is a big mental difference between turning over on purpose and turning over out of control. The goal is to develop a mental grace under pressure.