This glitch works as to getting around a death barrier on fallen. From here the Infected can throwing knife you, but cannot knife you from this spot, so it is also a good spot to go prone and hide. To start off, go to around the back area of the map. From here you should see a flight of metal stairs and continue to proceed up them.
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Climb up onto the white railing and look down. If looking down correctly your crossharis should be around this area.
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Now, from here it gets a little bit harder. From here you are going to have to strafe jump from the railing to the right side of this pole.
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Landing maybe tricky, without hitting the death barrier but be careful. To strafe, run straight and at the last second move your crosshairs a bit to the left and aim for the right side of the pole as show above. From that pole, you're going to want to land to the righter side of the window. If done correctly, you should be past the death barrier.
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It may take a couple of tries, but keep attempting it in private match and you should get it. Thanks and I hope you enjoyed my tutorial.