Where it works:
Vacuuming your stomach is an isometric contraction and works on the transverse abdominal muscles behind your six-pack, supporting your core and posture by controlling deep breathing during power movements.
What does it help:
Vacuuming your stomach during exercise increases your flexibility and aides in building more muscular muscle strength. It is also responsible for back support.

Why use it:
Vacuuming the stomach muscles help you gain more control over your abdominal muscles, assist in stronger explosive lifts, better pelvic floor strengthening, and can be done daily.

How to:
1. Start by taking a deep breath inward.

2. Exhale all the air out through your mouth.

3. While you exhale, draw your upper stomach as far as it will go in and up into your ribcage, creating a ribcage lift. For beginners, hold it in for 5 to 10 seconds. It is important to continue breathing normally during the exercise, so don’t hold your breath.

NOTE:

You may also want to add a pelvic floor lift as you contract your abdominal muscles.
With time it will become easier to hold the stomach in and feel comfortable. Aim for 5 seconds to start!

Source:

https://journals.lww.com/nsca-scj/Abstract/2020/10000/The_Abdominal_Vacuum_Technique_for_Bodybuilding.14.aspx