The benefit 
The side-to-side jackknife pushup trains your chest and shoulders and forces your core to stabilize your body throughout the movement, says Robert Dos Remedies, MA, CSCS. By increasing the demand on each shoulder, one at a time, it also helps eliminate muscle imbalances. If you feel pain when you do a shoulder press, this exercise is not for you. 
How to do it 

Kneel next to a bench with your feet on the bench and your hands beyond shoulder width, on the floor. Push your hips to the sky so that your legs and torso form nearly a right angle. This is the starting position. Bend at the elbows to lower your head down and towards your right shoulder. Turn your head slightly towards that shoulder as you do this. Straighten your arms and repeat the move to your left. Start with 3-5 reps to each side. 

Expert's tips 
"You should feel like you're actually upside down while you're doing this exercise," . "Push your hips high so that your torso is almost vertical." Does this seem too difficult? 
Start with your feet placed on the floor or on a small step. Then, when you can do eight reps to each side, increase the challenge by placing your feet on a Swiss ball instead of a bench. 

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