The first proposition is "Someone is knocking on the door", though this propostion has not been uttered. (5a) [VP2 WH +Q [VP1 Neg C Prom T R Asp Vc V NP1 NP2 ]]. (5b) [VP2 WH +Q [VP1 Neg C Prom T R ...

Next, let's add [+WH] to (3): (4) [ +WH [ +Q [John buy something]]]. The WH-operator is modifying (3). [+Q] is now associated with the elicitation of determining what 'something' is: what John bought.