BNA has the MMW machines. Sometimes they are on, sometimes off. Even if they are on, they only direct a portion of the passengers through. If you enter security from the Southwest ticketing side, use the lanes to the right hand side for the TDC. These usually open to more lane options than the ones next to the falling fountains. They often have the rope lines set up so that you can cross over to any screening line you want, but sometimes not. In my experience, the middle of the security area gives the best chance of missing the MMW. If you get in a line where the MMW is being used, watch those going through the machine and try to time getting your stuff in the xray so that someone has just stepped into the scanner in front of you. They seem to run everyone through the WTMD until there is space in the scanner.

Using this technique, I have just had to opt out twice and that was because of my own stupidity and was not paying attention. I took 40 flights out of BNA last year, 20 or so since they got the scanners.