This is a small plastic strainer that will stop twine and 20mm fencing tape from sagging. It's small & lightweight so fairly unobtrussive.

You don't need to cut the fenceline as this will take the conducting line in the central mechanism and grip it for tensioning. There is a ratchet to prevent the line from unraveling which when pressed will release the line if you have to let more line free.

This tensioner will not store much line but is meant purely to keep the fenceline taught when it sags through expansion.