Root restriction: A restriction is imposed on the final root node of any XTAG derivation of a tensed sentence which equates the wh feature and the invlink feature of the final root node. If the extracted NP is not a wh-word i.e. its wh feature has the value -, at the end of the derivation, Sq.b:wh will also have the value -. Because of the root constraint Sq.b:wh will be equated to Sq.b:invlink which will also come to have the value -. Then, by ((576)), Sr.t:inv will acquire the value -. This will unify with Sr.b:inv which has the value - (cf. (577)). Consequently, no auxiliary verb adjunction will be forced. Hence, there will never be inversion in topicalization. If the extracted NP is a wh-word i.e. its wh feature has the value +, at the end of the derivation, Sq.b:wh will also have the value +. Because of the root constraint Sq.b:wh will be equated to Sq.b:invlink which will also come to have the value +. Then, by ((576)), Sr.t:inv will acquire the value +. This will not unify with Sr.b:inv which has the value + (cf. (577)). Consequently, the adjunction of an inverted auxiliary verb is required for the derivation to succeed. Inversion will still take place even if the extraction is from an embedded clause.
This is because the adjoined tree's root node will also have its Sr.b:inv set to -. Note that inversion is only forced upon us because Sq is the final root node and the Root restriction applies. In embedded environments, the root restriction would not apply and the feature clash that forces adjunction would not take place. The invlink feature is not present in subject extractions. Consequently there is no inversion in subject questions. Subject topicalizations are blocked by setting the wh feature of the extracted NP to + i.e. only wh-phrases can go in this location.