Bream will usually mouth the bait before seriously eating it. Have a look at their front teeth with the hard plate. When they feel any tension they will drop the bait. The slack line and running sinker means they don't feel as much and suck in the bait deeper so that you can get that hook past those front teeth and increase chances of a good hook set.
Bteam being finnicky feeders when they have the bait in the mouth and move off the line comes off the reel easy and they dont feel any pressure from the rod ,been using this method forever and works well