Lie down on the bench with your lower back pressed flat into the bench and your head and shoulders raised slightly above it.
Place your hands lightly on the sides of your head; don’t knit your fingers behind. Lift one leg just off the ground and extend it out.
Lift the other leg and bend your knee towards your chest.
As you do so twist through your core so the opposite arm comes towards the raised knee, and do The Bicycle Crunch