about the closed face generating more topspin
/o <---- imagine this as a racket with a closed face hitting a ball
and imagine this ---> lo as a flat face hitting a ball
how exactly does /o brushing up create more topspin than
lo brushing up?
Part of your shot directs the ball, while an other fraction directs the edge of the ball. As a matter of fact, you will produce spin by brushing up the back of the ball... the problem is that you will also cause the ball to lift: it will travel higher.
Your swing path prior ball contact mainly influences the ball's launching angle whereas the angle of the string bed mostly impacts the spin/pace ratio. As a fact, most professional forehands (prior contact) are swung nearly horizontally: between 10 and 20 degrees of slope from the horizontal axis, with notable exceptions such as Nadal who frequently reverts to more vertical swing, getting sometimes close to 30 degrees.
Using basic trigonometry, you can understand how much the racket goes up for each forward inch and that gives you an idea of how the linear and vertical parts of the vector share the total energy that is transferred to the ball.
A very obvious thing is that, until you reach 45 degrees, you get more forward action than upward action... so, more of your kinetic energy is directed forward. It could explain why it's better to use the tilt of the racket to accelerate the edge of the ball: with most of your energy being directed forward, you also get more spin by attacking the ball forward.
The tilt basically allow you to hit the upper edge of the ball, which creates spin. By swinging more vertically, you're essentially also pushing the ball upward. Although IT DOES affect spin production, it affects it marginally.