The Assault on the Control Room Shortcut requires very good timing, so it may take some practice before you can get it. First go on to the first bridge, and make sure you have a plasma grenade. Try not to wake up any Grunts, as they will be annoying and dangerous. Throw a plasma grenade on the ground, right next to a Shade. Right when the grenade explodes, quickly press X (E in Halo PC). If you're lucky, the shade will right itself and you will fall safely to the ground. By doing this, you can skip three quarters of the level.
At the bottom is a snowy and empty world. Events that are going to happen haven't happened yet, because the things that trigger them haven't been done yet.