The phrase "stomach vacuum" refers to a breathing exercise where you suck in your stomach as much as possible while exhaling deeply. It helps to strengthen your abdominal muscles and can make your waistline appear slimmer. Full definition
You can also work your transverses abdominis, to pull your waist in tighter, by doing stomach vacuums but I suggest you don't work your abs directly with isolation exercises, keep a strong core by performing compound movements and using free weights instead of machines. (fitbodyhq.com)
She incorporates HIIT, resistance bands, kettle bells, and stomach vacuums into some classes which really keeps things interesting, fun and effective. (golubkakitchen.com)
And to end I will give you not an exercise to burn fat or gain muscle but a good tip — A great way to get a flat stomach is to do stomach vacuums. (fitbodyhq.com)