If you use a glassless negative carrier and the negative heats up, the shape of the negative can become distorted. That's "negative pop."
Tilting the easel--Imagine that you are photographing a building facade straight on with the camera pointing up. The straight lines will converge at the top. If you had a view camera, and you wanted to have the lines appear square, you might raise the lens with the camera level to get the whole building in the frame with the corners square. If you don't have a view camera, there are two options. One is to level the camera, shoot with a wide lens, and crop--this would be the equivalent of using front rise on the view camera. The other is to tilt the easel in the darkroom, and this would let you get one plane square on the print--so it works for a straightforward building facade, but not for a complex scene with buildings at varying distances from the lens.
If you tilt the easel, though, you need more DOF to get the whole image in focus, so you can either stop down the lens, or if you have an enlarger with a tilting lens or neg stage, you can use the Scheimpflug principle to get the image in focus, like using tilt on a view camera. The principle is that the lens plane, the film plane, and the image plane all meet in a line (unless they are parallel). So if you tilt the easel, and you can also tilt the neg or the lens, you can make all three planes meet in a line, and bring the image into focus without having to stop down the lens.