An abdominal brace occurs when you have contracted both the abdominal, lower back, and buttock muscles at the same time.
Here is how to perform the maneuver:
Stand up straight and place one hand on the small of your back and one hand on your abdomen.
Bend forward at the waist and feel the lower back (extensor) muscles contract.
Come back to an upright posture and feel them “turn off.”
Without bending forward, contract the abdominal muscles (like you are about to get punched in your gut – feel them tighten with one hand) and the buttock muscles (as if you are holding in a bowel movement). You will feel the lower back muscles contract (with the other hand) when you contract your abs and buttocks.
Another way to feel the brace is to try coughing or blowing out as if you were going to blow out a candle. You will feel the contraction in abs, low back, and buttocks.