Get in a push-up position with your feet inside the straps. Your torso and legs should be straight and rigid.
Exhale as you flex your knees, hips and waist and draw your knees into your chest.
Inhale as you extend your knees, hips and waist to return to the starting position.
Repeat for the prescribed number of repetitions.
Comments and tips
If the waist is flexed, the abs and obliques will be activated dynamically and will function as synergists. If the waist is not flexed, the abs and obliques will be activated isometrically and will function as important stabilizers.
Perform the suspended jackknife in a controlled manner. Do not swing or use momentum.