I think there’s a slight different between a movement and a stretch.
You don’t always need to tighten everything and stick to the movement principles when you stretch. You still shouldn’t put yourself in a compromised position – which is usually done by leveraging the floor, wall, or other object.
Yoga poses, such as the forward fold require some muscles to be relaxed and allowed to stretch and lengthen. I’m not very familiar with Cobra, so I’m not sure where this falls.