Your best bet is to watch the videos and to contact the benefits chair at SWAPA. He is the guru on all things regular plan. There is a slide show they put out last year at benefits enrollment time that laid it all out.
The way I understand it is this:
Regular plan is the primary payer. Tricare is secondary. As soon as you hit the cap for the regular plan, it takes care of everything 100 percent. The cap is $2,500.
Regular plan doesn't cover preventive, so tricare will in effect become the first insurer for everything preventive. There may be some phone calling and paperwork shuffling involved in making this happen as tricare is tricare and they will attempt to say that the regular plan should be paying first.
In the end, it is an EXCEPTIONAL value. The cost for this world class health insurance? Zero.
Due to my family size and makeup right now, I don't do this, but I plan on switching to it as soon as a few of my chicks fly the nest. Right now, I do tricare only with benefits plus vision and dental.