You don't the permission to view this video
Yes. You must first remove all existing water in the line before soldering. Water cools the pipe, which doesn’t allow it to heat up to the proper soldering temperature. A propylene torch can help with this because it burns hotter. The best measure to take is to remove all water from the pipe before soldering. Stuff a ball of white bread (Wonder Bread works great) into the pipe to stop water. Then, after you soldering the fitting and turn the water back on, purge the line of the bread through the nearest exterior hose bib. Plumbers have used this trick for years.