Hello, fitness question about the abs here?

How does one brace for a punch to the stomach? It's a well-known cue in the gym but it's very unfamiliar to me.

If it's an isometric abdominal contraction does that mean I can use other muscles isometrically to help fire the abs? For example: squeeze underarms (latissimi dorsi) and/or inner thighs (hip adductors)?

Please explain how to brace abs using the punch cue. Thanks!