It's burning all the carbon and varnish deposits that build up over time. I'm doing it to my 03 SR5 tomorrow and going to flip out the neighbors. There is a fire dpt. near my house so let's hope they don't see the smoke and come over here thinking I set something on fire!
I did it to my 03 Highlander base and the out come was better MPG, felt more peppy, and idled a lot better, it had if I had to guess about 106-108k when I did it.