From Tomb of Annihilation, the description of the magic dagger Bookmark (p. 206) says:
While you have the dagger drawn, you can use a bonus action to activate one of the following properties:
- Cast dimension door from the dagger. Once this property is used, it can’t be used again until the next dawn.
- Cast compulsion (save DC 15) from the dagger. The range of the spell increases to 90 feet, but it targets only spiders that are beasts. Once this property is used, it can’t be used again until the next dawn.
Both dimension door and compulsion have a casting time of one action.
Does using a bonus action to activate one of these properties merely enable you to then use your action cast one of those spells, expending both your bonus action and action, or does the Bookmark essentially change the casting time of those spells to 1 bonus action?