The underhand grip single arm tricep press-down is an outside the box tricep exercise that favors the long head. By doing this triceps exercise in a kneeling position, you eliminate a lot of momentum and cheating. Focus on squeezing the triceps hard at the bottom of each rep, and making sure your hands stay in a supinated position throughout both the concentric and eccentric portion.
Try 10-12 reps on each arm with 60 seconds or less rest between sets. Do 5 total sets per week at the minimum for tricep growth.