Lie on flat bench holding one dumbbell above shoulder, palm facing in.
Bend elbow to lower dumbbell beside head without moving upper arm.
Extend arm fully, contracting triceps at the top.
Dumbbell Single Arm Skullcrusher एक्सरसाइज़ गाइड - 上腕三頭筋 वर्कआउट | MuscleWiki