Opportunity – make sure your opponent is behind the baseline and / or off balance before you use this shot.
Disguise – Fed uses the backhand slice shot setup initially but changes it to the dropper.
Racquet path – the path of the swing is pretty much straight down to the ground with little or no forward motion. The idea is to absorb the pace of the oncoming ball to slow it down and have little velocity when it goes back over the net.
Underspin – With added underspin the ball lands just barely over the net with enough spin to make it come back to the net. This makes it more difficult for the opponent to reach the ball.