There is a technique to it, and yes the string is designed to "unzip" not to just be pulled out, I don't think I'd be strong enoungh to do that. My partner has been living with us for over a year now, and he still can't do it, despite me giving him lessons! He feels a right numpty!
Some feed sacks are easier than others to undo, but basically, always start at the end where the string is shortest. There will be a loop, with a bit of the string through it. All you do, it take out the bit of the string through the loop, and then pull the loop you have left, and viola, the bag unzips.
I might try and get some pics, because it is hard to explain.
Beth