Yes, he means that you set the die so it doesn't crimp the case ( raise the Presses ram with a 9mm case in it ,into the die & screw the die down until you can feel the crimp touch the case & back it off a little ) then seat a bullet , after you have your proper COL set , you back away the seating stem enough so you can screw the Die to set the crimp with out the seating stem seating the bullet any farther into the case , once the crimp is set , with the press ram still all the way up ,like your seating a Bullet & crimping the case around the Bullet ,screw the seating stem back down to make contact with the bullet & lock it down .
This will give you a fully seated bullet , with a crimp , just make sure your COL didn't change ( because the seating stem may not be far enough down , seat another bullet to see where its at ) & if it did move, adj. the seating stem correct it . Or you can set the die to only seat the bullet & Crimp in another step . Its what I do , but I also hand prime all my ammo .
The bullet pushing back into the case can be a weak crimp or your COL was too long for the chamber .
9MM should be a Taper crimp , because the 9MM 's case head space's in the chamber on the case mouth & it will not feel like a crimp , more like light swaging , because all your doing is closing the case around the Bullet . Just like the 45 ACP & a bunch of others.
Lee also has a Factory Crimp Die , that puts a decent Crimp on the Brass .It has its own instructions .