You have to be a spy to do this. On Dawn Waves you start out at the base and you go to the crane there will be 2 ziplines.
Take the far one and you will see a zipline under the far one.
What you do next is you hop on the 1st zipline and slide untill you get close to the 1 below you then you press b and you will be on the 1 below you (this is really good if their trying to snipe you).