Twist is an idea of machining timber a certain way that generates a twisted section of wood. Then designing tables and chairs that express and exploit that twist.
Stretton was trying to replicate the way Jean Prouve flattened out round tube to brace the curve of a chair frame in timber. With the terrace chair the timber section is deep where the moment in the joint is greatest and flat where the arm is supported and the moment is less.
Once the method and the proportion of the chair is solved, the rest is simply the technical resolution of webbing and finishing.