It takes the heat from the base, it goes into the heatpipes and the heatpipes carry it to the fins. Heatpipes are basically hollow metal tubes that have liquid in them, and they carry the heat up as it gets hotter, since heat is dense compared to cold. When the heat comes to the fins, it relies on the fan to carry the heat away. The SI-128 needs a fan, as ThermalRight heatsinks don't provide a fan. I would suggest buying a cheap Yate-Loon, Delta Tri-Blade, Panasonic Panaflo or Sanyo Denki.
Since Core 2 doesn't run too hot, you don't need to worry about a high-speed fan. A medium speed should be fine for what you're using.