Buying and selling.
I am still in the accumulation phase, and as an overanalyzer, I keep track of my AA weekly. I am nearly at 81% stocks and 19% bonds(and cash and remaining junk that Morningstar reports is in most index funds). I'm about to make my monthly aftertax deposit, which I'm limiting myself currently to either VTSAX and VTIAX. Also, I'm a smidge heavy on international versus US (desired 70US/30international, actual 69.81/30.19), so that means VTSAX. So I sold within my 401k some for VBMPX, and then tomorrow will buy VTSAX.
Overall, it'll still be a net increase in VTSAX.