The first time you cast an extra-fast action fly rod you may be a bit surprised. Having started like most folks throwing medium-fast and fast rods, it certainly was eye-opening to me! These rods are made to be powerful casting tools designed to conquer wind, bulky flies, and reach extreme distances. While these can be very good things, such attributes may come with one downfall: performance at shorter distances.
Let's say you have a very fast action 8-weight rod coupled with an 8-weight line. When casting a decent distance it will probably feel pretty sweet with lots of power and good feel. However, when taking short casts it may feel too stiff and dull. Why? The rod simply needs more line weight to help it flex at short distances. When throwing to a nearby target, the small amount of line you're casting doesn't weigh enough to optimally flex or "load" such a stiff rod, which in turn results in this lack of feel. At longer ranges, you have more line outside of the guides (more weight) which causes the rod to load more deeply, thus increasing the feel. How to improve performance at shorter ranges? Uplining can help!
Uplining simply means to use a line that is rated heavier (usually + one line size) than what the rod is labeled for. So, if you have an 8-weight rod you'd upline by using a 9-weight line on it. Since the head of this heavier line weighs more, it will cause the rod to load a bit deeper while casting at any distance. This pays big dividends at shorter ranges by increasing casting ease, feel, and accuracy. On the other hand, uplining may cause the rod to lose some of its "oomph" when casting very long distances since all of that heavier line weighs quite a bit. It all really depends on the line and rod.
Aside from its usage on just stiff rods, uplining can also be good for helping to cast bulkier flies and can improve efficiency when making short, quick casts to targets. While it's not for every rod or every angler, uplining definitely has its place and can be a real help in certain situations. Keep it in mind!