You need blood flow to go to ur brain because not enough is going to the brain that's why u get dizzy. When u flex ur stomach, ur blood rushes to the outer parts of body like the head, away from the core.
So for example if u are standing up, and feel dizzy, lie down so that blood goes to ur brain. If u are getting up and feeling dizzy it's cuz U got up too fast and u need higher blood pressure