With three muscles encapsulating the Triceps (tri=3), referring to the “outside of the triceps” is a curious question. By outside, if you mean not the back end of the arm, then the muscle you’re focusing on is the Lateral Head. Single arm reverse pull downs is a great way to target that muscle as well as keeping the elbow tendon from any possible inflammation. Many clients who peform supine (lying down on back) “nose busters” as they’re often called, use too heavy a weight, and flare out their elbows causing excess strain on that tendon.
By standing and using either a cable machine, free motion machine, or resistance band/tubing, you can keep a more stable arm and focus on the triceps. Use a reverse grip (underhand) on a cable handle, elbow bent, hand at chest height. Pull the cable downward, whilc keeping the arm tucked in close the body. Squeeze the muscle at the bottom of the motion (when arm is straight down at side), then slowly (fighting the resistance), bring hand back to starting position. 10-15 reps x 4 sets will fatigue muscle for proper toning/hypertrophy.