It involves the homophonity of 'scrute' and 'screw'.

As I recall, the riff uses the verbs inscrutable, impenetrable, and impregnable. Not all of which are equal opportunity, except in some outlying sections of fanfic where I do not care to go.

ETA: Particularly looking forward to the reactions to the current chapter, although I probably won't get a teaser post here till sometime during the weekend.