You can slightly bend your knees or have your legs extended fully.
Anchoring your feet/legs will create more tension in your quads and hip flexors.
Your arms are at your sides at the bottom position. As you raise up, they elevate overhead.
The arms passive.
Take a breath out and imagine your abdominals curling your torso into the top position. Similar in the manner of a bicep curl.
You don't want a rigid torso position. Your back should not be arched during the execution of the exercise.
Ignore the notion to raise your chest up to the ceiling.
Your core is not "braced" as if you are doing a squat or deadlift.
From an anatomy and biomechanics perspective, the rectus abdominus flexes the trunk.
You cannot flex a muscle if it is tense.
At the top position, you are "uncurling" your abs back to the floor.
Don't flop your upper back on the ground.
Don't hold your breath on the concentric or eccentric portion of the movement.
Breath out before your perform both the concentric and eccentric portions of the lift.
*Tips*
At first, your legs will want to elevate or "spray" out. Through practice and greater core stability, you will be able to execute without much leg involvement.
Create the mind-muscle connection that your legs are passive throughout the movement. Your quads and hips should not be taxed after each set.
Again, you are "curling" your body using your abdominals to complete the exercise.
Don't throw your elbows forward to elevate your torso.
