The key word is 'clean'. Unless all surfaces to be soldered are clean, and then there's flux, heat is not as critical (but it is still important) as you need it to get hot enough before you've boiled or burned out all of the flux for the solder to flow...heat with no flux allows corrosion which means a lousy connection. Heat the fitting (primarily), not the pipe.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014