What is abdominal bracing? It’s when all of your core muscles work together, a “Super Stiffness” occurs, and all 3 layers of the abdominal wall are activated to protect and stabilize the spine and discs.
Without bending forward, contract the abdominal muscles (like you are about the 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.
Here are a couple videos showing abdominal bracing and how it’s done!