It slows down the step, so the first time around, bd/3 would give the first third of the event. The trigger message gets sent at the start, so you hear it then. The next two times you get the second and third part o fthe event. There's no trigger message for those.
If / isn't clear by the end of week 1 lesson 4, let me know! https://club.tidalcycles.org/t/week-1-lesson-4-mini-notation-part-2/