Stand or sit with a weight plate in your hands, arms extended straight above your head
Lower the weight plate behind your head by bending at the elbows.
Keep your elbows close to your head and shoulders relaxed.
Push the weight plate back up to starting position, extending your arms fully.
Plate Overhead Tricep Extensionエクササイズガイド - Tricepsワークアウト | MuscleWiki