In layman's terms, it's actually easier to induce and reverse a magnetic field—which is how data is written and read in a hard drive—as the temperature increases. So TDK has developed a laser system to temporarily heat the area where data is being written, which in turn allows them to squeeze more bits onto a platter.
The breakthrough that made this new approach possible was the development of a laser, which TDK refers to as a near-field light generator, that creates a beam that's just tens of nanometers wide—any larger and you risk erasing data around the area where it's being written. And for comparison, that beam's about 1/10 as wide as the lasers used to read a Blu-ray disk—so we're talking seriously tiny.
The technology could very well be the next big leap needed to push hard drive capacities to the next level, and TDK is confident it could be implemented in commercial products as early as late 2015. More here.