The goal is to melt the ice without damaging the pipe.
If you notice water draining slower than usual, or if you hear strange bubbling noises when you flush the toilet, the freezing process may have already begun. If ignored, the situation escalates quickly. Once the blockage is complete, using any plumbing fixture in the house will cause water to back up into the lowest point in the home—often a basement floor drain or a shower stall.