To add clarification, potassium (or sodium) metabisulfite does not kill yeast. Neither this or potassium sorbate will stop a fully active fermentation. The purpose of adding a sulfite is to prevent the growth of wild yeast strains, bacteria, etc. It also works as an antioxidant. Sulfites are usually used as a preservative once fermentation is complete, not as a method to stop fermentation. Potassium sorbate is the compound that can be used to prevent fermentation from starting back up. But again, this isn't intended to stop an active fermentation or kill yeast. What it does is prevent yeast from growing and multiplying. So if you add potassium sorbate and then backsweeten, there may still be a very small amount of fermentation that occurs but it will be essentially insignificant.
As majorvices mentioned, these two chemicals are much more often used in wine and mead. I've only used them in beer once. I had a brown ale that I wanted to backsweeten with honey, so I used metabisulfite and sorbate to stabilize the beer, stirred in the honey, and kegged it. The keg was refrigerated so the fermentation wouldn't have picked back up, but I was planning to bottle part of the batch and didn't want there to be problems if a bottle warmed back up to room temperature. The process of stabilizing and backsweetening works fine for beer, but it's not something you really hear about.