I'm wondering whether Betfair are still playing with it...
My experience is...
After 2pm my triggers would have anything under 80p rejected. I confirmed this with the betting interface, using a wide range of prices. Then between 6pm and 8pm, anything under £1.60 was rejected, again using different prices with the betting interface. Then after 10pm, my triggers were ok with anything above 80p again. That's when I turned my triggers off.
BTW I always back first, then trade out with a lay. When I was using the betting interface, I was just laying from the outset, but the results were the same as the triggers after a back bet.
Now after Oxa's post it seems to have changed again.