When I use the term controlled feeding, I do not mean to imply that the cartridge is under the control of the extractor throughout the forward movement of the bolt, but that it goes from the control of the magazine into the control of the extractor without interruption.

The chief virtue of controlled feeding is that it virtually eliminates the chance of double feeding, that is, the possibility that should the bolt be drawn back before the extractor has engaged, but after the cartridge is free of the magazine, there is the possibility of a second round being fed from the magazine when the bolt is moved forward again, a quite difficult situation to clear, since the second round will have to be removed before the bolt can complete seating the first cartridge in the chamber and engaging the extractor. Since the second cartridge will not have engaged the extractor either, removing it requires the shooter to give his full attention to removing the loose cartridge with his fingers, not a happy situation to be in when a quick second shot is called for.

This may seem far fetched, but I have seen it happen on the firing line in a rifle match. In my experience, if something can happen, it will happen, and at the worst possible moment.