Not a very frequently used technique but it's the most effective one. It's not frequently used because it requires extra money that people aren't usually willing to spend. The flash part in this technique refers to the closed-cell spray foam that's sprayed on the outside of the house.
Now flash and batt have a bunch of benefits as you're taking advantage of two different products, two different insulation products i.e. spray polyurethane foam and then a traditional bath on the inside. You do this for a couple of reasons. The spray polyurethane foam is a great air sealer, it adds a bunch of structural integrity to the house but like said before it's a pretty pricey endeavour to get into spray foam.
Especially, closed-cell as it's the most expensive insulation you can use in your house. And beyond that 1-2 inches depth, there may or may not be any benefit of filling a cavity with a closed-cell. What you do is finish sealing off the cavity with a traditional batt product. The batt chosen is glass wool that's treated with fire retardants. By installing the batt over the thermal blanket insulation we get a big benefit for fire resistance, the other big benefit is soundproofing as it's not only now used on the exterior walls but also the inside walls of the house. And who doesn't love a quiet and peaceful house?
There you have it, this is the benefit of paying a little extra for flash and batt insulation.