During a presentation on injury prevention, I taught a group of personal trainers how to instruct transverse abdominis contraction. Check out the video:
VIDEO TRANSCRIPTION (Please excuse grammatical errors, as this is simply a transcription of conversational speaking)
If you palpate your anterior iliac spine and just go with your fingers to the soft tissue right inside of it, you bring your belly button toward your spine. If you’re doing it right you’ll feel your transverse abdominis, your deepest abdominal muscle, great stabilizer of the spine. It’ll firm up and push out into your fingers and that’s before I’d have them doing back extensions. I’d make sure that they have a really solid contraction of that, that they can do it without holding their breath and hold it for twenty seconds at a time. I’d make sure those baseline things are there before I have them doing that but it’s a good exercise as well, just when they’re ready for it.