In a calibrated oven.

I don't think that there can be a very wide temperature band between "too cold to fix" and "components starting to fall off" for a two-sided board, the temperature band might even be non-existent. (Or is surface tension a function of temperature, enough to hold components at low temps, dropping when hotter?)

But professional repair shops are using a more sophisticated form of the same method. I wonder if the have some kind of adaptive support layer instead of a simple grille or flat surface?