It is usually depicted by the person with their bottom completely off the floor and whilst this is an advanced level of the exercise people will tend to cheat by swinging their legs into this top position (which I have an example of further down in this post.) The key is to straighten your legs and without using your arms as leverage try to get your bottom just off the floor by a few centimeters! It is a subtle exercise to be able bring the right muscle groups (of the pelvic floor and the lower abdominals) into play.
So to to get this action right, start by lying on your back on the floor and your head in line with your body. Your legs should start the movement with your feet pointing to the roof (or sky if you like) and your knees bent. Your arms lay on either side of your body and your hands palms down, fingers extended but relaxed. All you need to do is to straighten your knees and endeavor to get your bottom off the floor as an extension to straightening your knees. Don’t swing your legs in anyway and don’t push down with your arms – just squeeze your pelvic floor muscles and lower abdominals so your bottom actually curls off the floor. It is this subtleness of the movement that is the hardest part to perfect. If this doesn’t happen, then the power of the exercise and the reason for doing it is lost! Hold this top position for ten seconds before lowering your bottom back down to the floor. Because of the intensity of the movement you only need to do one set of ten repetitions to gain maximum benefit from it.
You may not be able to get your bottom off the floor to begin with. Don’t concern yourself with this as you will feel the right muscles working if you do this exercise properly! It will take time for some people to perfect this movement, but this is usually due to their lack of mobility in the lower spine and lack of flexibility in the hamstrings muscles (back of the leg).
Now here is a depiction of what most people see as doing the exercise properly…just understand that this version is really a whole different exercise!