Adjust the carriage so that you can place your bent legs up against the bar.
Lie on the ground with your legs bent, hip-width apart, under the bar. The arms are resting on the floor, relaxed at the side of the body. The thighs touch the abdomen. The feet press against the bar at hip width.
Press the bar upwards evenly with the legs and the balls of the feet until the knees are almost fully extended. Keep the buttocks flexed and lower the legs to the starting position again, knees touching the chest.