Raj Rishi

Here is a simple 3-5 minute exercise called "ab vacuums". It may sound a little weird but can flatten your belly provided you do it consistently. 

The best thing about it is that you can do it anytime anywhere. You can do it standing or sitting or even while driving a car, provided, of course you keep your eyes on the road and hands on the wheel.

This exercise, however, does not result in burning the fat to flatten your belly as most other exercises are aimed to do. On the other hand, it strengthens your deeper transversus abdomins by pulling in the lazy stomach also popularly called beer belly or pooch belly.

What are transversus abdomins?

The transversus abdominis, also known as the transverse abdominus or transverse abdominal muscle, is a muscle layer of the front and side of the abdominal wall. It goes deep to the internal oblique muscle or the intermediate muscle of the abdomen and regarded a significant component of the core, i.e., the body minus the legs and arms. (Wikipedia). The exercise can also support a healthy back.

Exercise

Ab vacuums, as the term denotes, involves pulling the belly button in which results in  vacuuming the abdomen by sucking air out of it. As mentioned above you can do it sitting, standing, kneeling in a four-point position, or even lying flat on the floor.

Here are just 2 simple steps:

1. Pull your belly button as much in as you conveniently can by imagining you're trying to touch your belly button to your spine and holding for 10-20 seconds at a time.
2. Start by inhaling deeply. Then, as you exhale, start pulling the belly button in towards the spine and hold it there for 10-20 seconds while just taking short breaths. Repeat for several of these 10-20 second holds, perhaps 5-10 times.

Here is a video for detailed performance

Source Here