That’s the cue when doing compund movements. Tigthen your abs, not exactly like flexing them, but more like stabilizing the entire midriff, not allowing it to bend to any direction. Important for safety and taxing for your core as well, providing yet another reason for doing compounds.
The reason why stabilizing is so effective at developing core muscles is because rectus abdominis is not the only muscle you have on your stomach. Behind it is another layer of muscles called transverse abdominis, which is responsible for protection during those movements. A lot of people care a lot about how their stomach looks and doing compunds as well as other stabilizing exercises like planks and using the ab wheel will push the rectus abdominis out due to enlargement of the transverse muscle.
Another common misconception regarding stomach aestethics is the bewildering belief that by doing a lot of exercises that target it you can locally shed fat. There is no way to lose fat of a single body part with another method than liposuction. During a caloric deficit – hence the saying “abs are made in the kitchen” – your body uses the accumulated fat as fuel and it burns it throughout the entire body. By doing thousands upon thousands of crunches you will not see visible results, because the muscle is covered by fatty tissue, if you want to lose that then examine and adjust your diet.
Now regarding that saying – “abs are made in the kitchen” – I disagree. They are made during training, what you do in the kitchen simply reveals them. I know the saying is figurative, but it can lead to confusion. The look of strong, hard muscles doesn’t boil down to simply eating less and sitting on your ass. An easy way to develop them is by doing compund movements and maybe incorporating from time to time an isolation exercise. Squats and deadlifts – due to their stressing nature – are perfect for this. Form is paramount, and good form means you are bracing your core throughout the entire movement – effectivly training your abdominal muscles.