BT RH 7b(29) : כי דקתני מידי דחייל מאורתא, מידי דלא חייל מאורתא לא קתני‏ : that is when he is teaching about something that takes effect in the evening; he is not teaching about something that does not take effect in the evening
See the entire entry for ḥwl#3 vb. to fall upon destructively