This is one of the most important design aspect of any sleep system that connects around the sleeper, such as a sleeping bag or a quilt that connects with straps to a pad or itself. By definition, it is when the inner and outer shells of a sleep system are cut to different sizes so that all of your down fill can stay fully lofted around you even if you push against it or pull it tight. If the inner and outer shells are not cut with a differential, any pressure the sleeper puts on it will compress it, compromising warmth greatly. If you pull the sides of a quilt not cut with a differential, ALL of the down along that line across will flatten out. Pull the sides of a differential cut quilt and all the of the load is received by the smaller shell and the down stay fluffy, like it should be. The fetal position sleep systems use a very large differential cut that is actually the full circumference needed to accommodate the target loft. So if a sleep system’s target loft is 2″ the outer shell will make a circle that is 4″ larger in diameter than the inner shell to accommodate 2″ of loft all the way around. This provides lot’s of loft space and eliminates any compression from shell tension. The fetal position systems are more likely to have more tension placed on the shells when the sleeper curls up inside and therefore require a larger differential. The mummy shape systems and quilts use a smaller differential since there is less tension placed on them.