Some bags have this 'feature', so that, apparently, you can move the down about so it's just right temperature wise.
I honestly can't ever see myself doing this. If anything I imagine if it's easy to move the down around the down would migrate away from the top, and if it's hard to move the down around I'm just not going to do it because I have better things to do.
"Secondly, the bag has a novel design using vertical baffles (the tubes that hold the down in place). They run up and down the body, rather than the more normal across. This stops the down from migrating towards the sides of the bag and creating cold spots over your chest. To stop the down migrating toward the feet in the vertical baffles, Marmot have put in internal baffles to keep the down in place within the longer tubes. Marmot calls these 'flow gates' and say that if needed you can actually move down through them manually – if cold spots occur. Personally, I trust Marmot's designers to know where the down should be distributed and can't see myself trying to move it around inside the bag, but it is nice to know you can if over the years you find cold spots. The vertical baffles are also said to involve less sewing than the normal horizontal ones, again minimizing weight." http://www.ukclimbing.com/gear/review.php?id=3605