Grab the bench from behind as you extend your legs and lean back as far as you can go, try to keep your legs straight and knees tight. Quickly bring your chest and legs together to a V shape position, Inhale as you extend the legs, exhale as you bring your knees in, don't move your legs only. You can challenge oblique muscles by twisting your legs to the sides, harder variation is to let go of the bench balancing on your pelvic bone and perform the same movement hugging both knees as you come forward and flying your arms to the sides for balance as you lean back.