For a Tricep Dip, you want to face away from a raised platform, with your arms shoulder width apart and held onto the platform.
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You can use a bench, side of a bed, coffee table, chair, step, box or a specific tricep dip rack.
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From here, starting with a straight arm, lower your hips as you bend your elbow towards a 90 degree angle, before then pushing through the chest, shoulders and tricep to return to a straight arm.