Of course it moves in an arc. But, the thought process is to get it down quickly from above. It doesn't literally go straight down, like chopping wood, but the thought and feel is to take it forward. The rearward arc just happens as you said in the other post. The shoulders tilt and turn to get the barrel flat. And, the tilt and turn is mostly instinctive, but in some cases it must be taught, because if you don't tilt them as they turn, then you do wind up with very little arc and a chopping down through the ball's path.
What I have discovered with teaching tilt. It only has to be taught because someone taught it right out of them by telling them to keep their shoulders flat or don't drop your back shoulder.