[1] Common bases of contingent or derivative liability by which third parties may be impleaded include indemnity, subrogation, contribution, and warranty.
For example, in a case where a driver rear-ended another car due to faulty brakes, and is sued by the accident victim, the driver may decide to implead the repair shop where the brakes were worked on because the driver's liability derives from the repair shop's liability for their faulty repair of the brakes.
Impleader is available only to defendants, not plaintiffs, unlike the similar interpleader action.
While many kinds of civil procedures devices occur in the form of motion, an impleader action is technically its own lawsuit.
Rule 14(a)(4): Any party may move to strike the third-party claim, to sever it, or to try it separately.