I learned this in a hospital so it is based on science and has to do with the vagal nerve which has a connection to the sympathetic nervous system, that fight or flight anxiety feeling.

So best to do this standing. Only 3 breaths so take your time. Open your mouth slightly and breathe deeply in through your nose. Pull the breath so the diaphragm goes as far down as possible. Visualize pulling it all the way into the pelvic floor and at the end when you can't hold it any more, squeeze the pelvic muscles like a keegle.

Then slowly blow the air out through your mouth until you are almost not breathing. Then start a new in breath. Repeat 2 more times. Careful not to get dizzy. I don't but some do. It is important to focus on the pelvic region and pull the breath down as deeply as possible. The vagal nerve ends at the pelvic floor and the muscles contracting help stop the panicky reaction.