Voiding is a major issue in the area of thermal management because air is a terrible conductor of heat. Anytime you have air entrapped in your thermal interface material, you're going to get decreased performance of that device. Grease is a commonly used thermal interface material and over time, because of thermal cycling and CTE mismatches, that grease tends to pump-out creating these air pockets, or voids.
To improve the performance and reduce the voiding or air pockets you get with the grease, you need a solid material. Heat-Spring is a metal thermal interface material that is compressed between the device and the heat spreader. Because it's a solid piece of material, it's not prone to pumping-out like a grease would. It also, because it's metal, has a very high thermal conductivity.