The control tether is a Droid weapon designed and created by the Droid Krunk, the weapon is used to take control of other Mils with an open data port (typically located on their back). The control tether has three levels, the first being the slowest to lock on, the slowest to reload, and have a fairly limited range at which the tether can shoot. These all increase as the weapon becomes upgraded.
In-game description (At level 3): "The elite tether operates rapidly and reconfigures the target Mil armor to be very strong Fire when the green lock-on bars appear." (Some weird wording on that...)