HeatSync syncs the air*, cpu and hdd temperatures and with the rpm of their corresponding fans increasing/decreasing one as the other is raising/dropping.
The machines supported are as follows : iMac's since v1.0 (might also work with Mac Pro), MacBook Pro since v1.1, MacBook Air since v1.2 (might also work with MacBook and MacMini).
The thresholds are the follows :
Ambient Air: low temperature 20°C-24°C, medium temperature 25°C-29°C, high temperature 30°C-35**°C
HDD: low temperature 45°C-49°C, medium temperature 50°C-54°C, high temperature 55°C-60**°C
CPU: low temperature 40°C-49°C, medium temperature 50°C-59°C, high temperature 60°C-90**°C
Ambient Air: low temperature 25°C-29°C, medium temperature 30°C-34°C, high temperature 35°C-40°C
Memory/SDD/HDD: low temperature 40°C-49°C, medium temperature 50°C-54°C, high temperature 55°C-60°C
CPU: low temperature 50°C-64°C, medium temperature 65°C-79°C, high temperature 80°C-90**°C
* the specific fan that is synced with the air temperature depends on the machine, alternative fans that have a outside intake like the optical one could be used, having air temperature synced could have no benefits, if in doubt leave it checked off.
** is roughly the manufacturer's maximum value for operating temperature
HeatSync is also available as MagicPrefs plugin the source code is open source.
My experience points in the direction that OSX management of the fan speeds is faulty on some machines, a iMac i have at least would shut down from the HDD temperature threshold reaching it's maximum and the whole time the HDD fan would run on idle, how wide spread is this issue is unknown to me, i am looking forward to relevant feedback.
It is a known fact that heat is one major factor lading to HDD failure, also there is some noise recently about the 2011 MacBooks shutting down abruptly from what looks like reaching the maximum temperature threshold with the fans only kicking in just before it shuts down, so HeatSync might provide some proactive aid.
However i do not guarantee anything except the initially stated that the fan speeds will be kept in sync with the temperatures, if your machine burns and explodes or otherwise malfunctions as a result of using this application i can not be held responsible for it.