Conjunctive waaw combines two elements in a statement; circumstantial waaw relates one act to the condition existing while it occurs. Its function depends on syntax and contextual suitability, not on the letter's shape alone.
For instance, “دخلت وأنت صائم” connects entering with the circumstance of being in a state of fasting. It does not mean that fasting necessarily started after the entrance.
Where the surrounding wording genuinely establishes the circumstantial function, a simultaneous condition may be inferred. That function should not be imposed upon every waaw without evidence.