NEVER cut the Slackline when there is tension in the system. Spider Slacklines has a solution for this situation: in the flat ratchet there is a hole for the emergency release. Take tight the ratchet handle with one hand; at the same time, puts a lever (like a screwdriver) in the hole and side pull until you release the ratchet.
WARNING! Remember that this is an emergency operation and should NEVER be done with the high tension of a 2 ratchets Trickline.
With Soft Release you will avoid this problem.