I come accross this thing first time just now at pt.wp, where I’m less active than in Commons (where it is not yet deployed). So I applied a few filters, and then tried to find an "OK" or "submit" button. Turns out there’s none — filter implementations happen on the fly.
This was a feature of the old Special:Watchlist which was changed a few years ago, with the addition of the "show" button, in the box labelled "Watchlist options". Back then a user sort of complained, saying he liked to tick the checkboxes on and off and get immediate results and now, oy vey!, an extra click is needed. This user was severely mocked and shown xkcd 1172 and yet here we are again, back to the old UI paradigm, with changes applied on the fly. (Of course the new UI’s elements are now very huge because reasons.)