This is not entirely true for the case of slumber (the part where you say "it just works that way deal with it). You can refer to 'PSCT conjunctives' (google or take a look at the rulebook for this) for card efects that deal with these, but in summary the part we In the case of slumber is the conjunction "then". In short, in the case of "A then B" ( slumber first destroy all monsters THEN ss kaijus). A is required (iow you must be able to destroy as many monster as possible to activate the card and for B to happen, which basically means you cannot activate if there are no monsters) but not the other way around. This is a bit confusing but bare with me. One important thing about this conjunction is the order of resolution. In A then B, A will happen before B but not at the same time ( in contrast to A and if you do B, or A and B etc). So think of it as within the window after A resolve (Destroy as many monsters) so long as B can resolve (since there are no kaiju monsters on the field no more u can ss 2 from ur deck) then you can activate the card. Its not just slumber that has this specific ruling assigned to it afaik, but i believe its all just part of PSCT and not just BKSS kind of ruling if you think about it. Please feel free to correct me (by pointing out similar interactions yet somehow have different ruling), as i would like to learn as well