A CPU's heatspreader is normally attached directly to the core by use of a paste or glue that has been enriched with micrometer-sized ceramic or metal particles.
It's this bond that is the root of the inefficiency, which IBM claims impedes heat flow up to 40%, due to uneven spreads and “clumping” of the particles:
In its current form, the process is quite inefficient: IBM's says that up to 40% of a CPU's total thermal budget (i.e., the cooling capacity available to draw heat away from the core) is consumed by these particles. This inefficiency is made worse because the particles aren't truly spread evenly throughout the paste.
A “cheap” manufacturing change has allowed IBM to fix this, incorporating trenches at points where it would normally clump up. IBM claims paste thickness required to properly cool a CPU can be cut to a third, and that the amount of pressure needed on a CPU to pull heat away could be cut in half. Anyone who broke a P3 or Athlon CPU when they had exposed cores knows how frustrating the large amounts of pressure can be. Sadly, there's no immediate benefit for desktop users. It is an IBM development, and whether or not Intel and AMD will see it is a worthy change remains to be seen. Given that it is being called a “cheap” improvement, we can hope – and with the importance that proper heat transfer is, there's no disadvantage for them in the long run.