The transverse abdominal muscle (aka TVA) is an often neglected muscle. I assume because a lot of the popular recommended ab exercises you see in magazines are mainly targeting other muscles of the abdominal wall such as the external oblique, rectus abdominis, etc. The TVA is the deepest of ab muscles.
Here is a list of TVA exercises so you can train that deep TVA muscle to obtain a tight tummy click on each one to see a video too:
I recommend training abs minimum 1x/week merely to assist in strengthening the lower back and preventing injury. I currently train abs only 1x/week because I power lift and this requires a lot of core engaging. Therefore, I’ve built up a very strong core from simply weight training heavy. However, I do not neglect ab training because it still is a muscle and needs to be stimulated.
I hope this helps .