Ab Exercises - Vacuum Contraction:
This exercise is very unique among exercises for your stomach and is one of the exercises for your stomach that most people have never performed.
Because the inner unit muscles attach from the pelvic floor to the lower spine, you'll feel a burn from front to back.
Again this, as with the other abdominal exercises, looks simple but its very challenging. It may take you a few weeks to be able to hold your abdominals for 90 seconds but it's worth it.
The vacuum pose is an isometric exercise that is done by pulling the bellybutton back towards the lower spine during a deep inhalation that is held for 15 seconds. The exercise is repeated for 3 to 5 minutes several times a day.
This pic sums it up best.
Now i know many women will be looking thinking 'jes i dont wanna look like that' trust me you wont. BUT practising this can help strengthen the stomach muscles giving you better control of them and inturn a stronger a visually smaller waist!