On many levels there is a floor underneath the real level, which normally kills you if you touch it. There are two ways of becoming immune to the kill floor.

Firstly, if you can "step" down from a ledge just above/below/at the same level as the kill floor, with the Boost" or "Power Jump" skills active(aka you need to be holding "v" or space.) Doing either of these before you get onto the kill floor will make you immune, however once on the kill floor, you can't jump at all, so in order to get off you need to find another ledge to climb onto without jumping.

The second way allows you to jump from any height onto a kill floor, and also negates all fall damage, however it only works with power jump. Before you fall off the higher ledge, hold space to activate Power Jump, then press shift to start sliding. Doing this means that you won't actually slide, or use the power jump ability once you reach the ledge. Instead, you will fall off the ledge just like normal, and land on the floor below. You won't take any fall damage, and you won't be killed if it is a kill floor. You can then let go of shift, but you need to keep power jump active until you are off the kill floor. Just like with method one, you can't jump on the kill floor, and will need to find a non kill floor to get on.