A flat serve hit with deadball might be considered a "topspin" serve, like a knuckleball in baseball. Low rackethand at trophy, finish after ball strike with a high hand and high elbow when the racket is pointing straight down at the ground.
The normal top/slice second serve uses some amount of pronation, but since most players hold the grip more backhand than for flat serves, the pronation never hits the ball flat. Ball toss is over your head, so the racket is angled, not pointing straight up at the sky. The angle allows you to swing up and over the ball, giving the topspin.