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Text tutorial:
1. Have a friend (preferably with a zombie shield) close to the desired area
2. When the zombie is attacking your friends shield, knife and jump. (you can knife and jump at the same time, it just depends what works best for you)
3. If done correctly, you will go straight up in the air. In some other cases you have to aim for the desired barrier.

Glitches found by s1opx