Actual definition: Giving your belly some room. After eating a lot, feeling bloated, but satisfied. You cannot move for a while and have to loosen the button of your pants in extreme ausbauch-mode and you make sounds like "pfff...that was good".
Basically you just take a rest (in the position you're in on that moment) before going to the next activity. You can not do ausbauchen when you haven't eaten. You can do it in several positions, most popular are sitting and lying (on your back or course, otherwise your belly will be sufficated). People who are trying to lose weight usually don't get to the state of ausbauchen.
Important note: It's not the same as being pregnant
Yum, that was good! Now it's time for ausbauchen.