[BrianG]

It's where you remove the internal thermal pads that hold the FETs to the case, and replace the heatspreader with something a tad thicker, and thermal epoxy the internals to the case. It drastically reduces the thermal resistance from my experience, so it heats quicker and cools off quicker. This makes any additional heatsink operate much more efficiently, but may void the warranty if you try to send it in for service.