The Jackknife exercise works your shoulders, triceps and abs.

[MUSIC] This move starts with you seated on the box with your hands right outside your hips. From there slide your hips out off the box, lower yourself down using your tricep. As you go down bring your knee up. You are gonna alternate knees as you go through each rep. Great. Keep your elbows in as you go down and keep that knee up. Good. [MUSIC]