From Rio:
Density Compensation
The majority of modern fly lines feature a taper at the front end, which ensures that they cast smoothly and land on the water without too much impact. With sinking lines and sink tips, this taper creates a small problem; as the line gets thinner the amount of coating on it gets less.
The coating is what makes a fly line sink, so as the coating is reduced, the sink rate slows down. Usually this means a tapered sinking line sinks in a curve - with the thicker, heavier and denser body section sinking faster than the tip - not good for controlling the depth, and especially not good for detecting a take, or setting the hook.
RIO's Density Compensation adds a more dense material to the thin tip, which ensures the line sinks straight - better for knowing when a fish takes the fly, and far better for setting the hook. All RIO's premium tapered sinking lines and sink tips are Density Compensated.