Do I need to use the tether strap when installing the seat forward-facing?

Yes — the tether strap must be used whenever installing this child restraint in the forward-facing position, whether using Lower Anchor Connectors or the vehicle seat belt. Connect the tether hook to the vehicle tether anchor behind the vehicle seat and tighten securely. If the tether strap is too short, contact Evenflo ParentLink at 1-800-233-5921 (U.S.) to obtain a tether extender. (Pages 44, 49)