Tethers are perfectly legal in SwimRun events and sometimes even mandatory, most of the time though it is an option that each team has to decide upon. If the run course is technical teams often will forgo the tether, but during flatter and faster sections the tether is frequently seen in action.
The tether however shines the most during the swims and will balance out swimmers of different calibers and speed. A 1:40 per 100 meter swimmer can easily stay in the draft of a 1:20 per 100 meter swimmer and sometimes the differences can be even more dramatic.
But tethering should be practiced and practiced often, because otherwise on race day it can become a headache.
You can create a DIY tether with elastic bungee cords and carabiners, but the Danish SwimRunShop.com offers a complete and very clever tether system that is ready for action out of the box. Plus you can also fly your country flag.